diff --git a/Android/jni/jnihooks.c b/Android/jni/jnihooks.c
index 2960c8ba..4bcf6b35 100644
--- a/Android/jni/jnihooks.c
+++ b/Android/jni/jnihooks.c
@@ -260,8 +260,8 @@ void Java_org_deadc0de_apple2ix_Apple2Activity_nativeOnQuit(JNIEnv *env, jobject
 
     LOG("...");
 
-    c_eject_6(0);
-    c_eject_6(1);
+    disk6_eject(0);
+    disk6_eject(1);
 
     cpu_resume();
 #endif
@@ -312,10 +312,10 @@ void Java_org_deadc0de_apple2ix_Apple2Activity_nativeChooseDisk(JNIEnv *env, job
     int ro = readOnly ? 1 : 0;
 
     LOG(": (%s, %s, %s)", path, driveA ? "drive A" : "drive B", readOnly ? "read only" : "read/write");
-    if (c_new_diskette_6(drive, path, ro)) {
+    if (disk6_insert(drive, path, ro)) {
         char *gzPath = NULL;
         asprintf(&gzPath, "%s.gz", path);
-        if (c_new_diskette_6(drive, gzPath, ro)) {
+        if (disk6_insert(drive, gzPath, ro)) {
             char *diskImageUnreadable = "Disk Image Unreadable";
             unsigned int cols = strlen(diskImageUnreadable);
             video_backend->animation_showMessage(diskImageUnreadable, cols, 1);
@@ -331,7 +331,7 @@ void Java_org_deadc0de_apple2ix_Apple2Activity_nativeChooseDisk(JNIEnv *env, job
 
 void Java_org_deadc0de_apple2ix_Apple2Activity_nativeEjectDisk(JNIEnv *env, jobject obj, jboolean driveA) {
     LOG("...");
-    c_eject_6(!driveA);
+    disk6_eject(!driveA);
 }
 
 // ----------------------------------------------------------------------------
diff --git a/src/disk.c b/src/disk.c
index 9862040a..b0211cef 100644
--- a/src/disk.c
+++ b/src/disk.c
@@ -403,6 +403,7 @@ static void denibblize_track(int drive, uint8_t * const dst) {
                 offset = 0;
             }
         }
+        assert(sector >= 0 && sector < 16 && "invalid previous nibblization");
         int sec_off = 256 * disk6.disk[drive].skew_table[ sector ];
         denibblize_sector(work_buf, dst+sec_off);
         sector = -1;
@@ -669,10 +670,9 @@ GLUE_C_WRITE(disk_write_latch)
 
 // ----------------------------------------------------------------------------
 
-void disk_io_initialize(unsigned int slot) {
-    assert(slot == 6);
+void disk6_init(void) {
 
-    /* load Disk II rom */
+    // load Disk II ROM
     memcpy(apple_ii_64k[0] + 0xC600, slot6_rom, 0x100);
 
     // disk softswitches
@@ -694,9 +694,7 @@ void disk_io_initialize(unsigned int slot) {
     }
 
     cpu65_vmem_w[0xC0ED] = disk_write_latch;
-}
 
-void c_init_6(void) {
     disk6.disk[0].phase = disk6.disk[1].phase = 0;
     disk6.disk[0].track_valid = disk6.disk[1].track_valid = 0;
     disk6.motor_time = (struct timespec){ 0 };
@@ -705,7 +703,7 @@ void c_init_6(void) {
     disk6.ddrw = 0;
 }
 
-const char *c_eject_6(int drive) {
+const char *disk6_eject(int drive) {
 
     const char *err = NULL;
 
@@ -729,11 +727,11 @@ const char *c_eject_6(int drive) {
     return err;
 }
 
-const char *c_new_diskette_6(int drive, const char * const raw_file_name, int force) {
+const char *disk6_insert(int drive, const char * const raw_file_name, int readonly) {
     struct stat buf;
 
     if (disk6.disk[drive].fp) {
-        c_eject_6(drive);
+        disk6_eject(drive);
     }
 
     /* uncompress the gziped disk */
@@ -769,22 +767,22 @@ const char *c_new_diskette_6(int drive, const char * const raw_file_name, int fo
 
     if (stat(disk6.disk[drive].file_name, &buf) < 0) {
         disk6.disk[drive].fp = NULL;
-        c_eject_6(drive);
+        disk6_eject(drive);
         return "disk unreadable 1";
     } else {
-        if (!force) {
+        if (!readonly) {
             disk6.disk[drive].fp = fopen(disk6.disk[drive].file_name, "r+");
             disk6.disk[drive].is_protected = false;
         }
 
-        if ((disk6.disk[drive].fp == NULL) || (force)) {
+        if ((disk6.disk[drive].fp == NULL) || readonly) {
             disk6.disk[drive].fp = fopen(disk6.disk[drive].file_name, "r");
             disk6.disk[drive].is_protected = true;    /* disk is write protected! */
         }
 
         if (disk6.disk[drive].fp == NULL) {
             /* Failed to open file. */
-            c_eject_6(drive);
+            disk6_eject(drive);
             return "disk unreadable 2";
         }
 
@@ -801,25 +799,29 @@ const char *c_new_diskette_6(int drive, const char * const raw_file_name, int fo
     return NULL;
 }
 
+void disk6_flush(int drive) {
+    // TODO ...
+}
+
 #if DISK_TRACING
 void c_begin_disk_trace_6(const char *read_file, const char *write_file) {
     if (read_file) {
-        test_read_fp = fopen(read_file, "w");
+        test_read_fp = TEMP_FAILURE_RETRY_FOPEN(fopen(read_file, "w"));
     }
     if (write_file) {
-        test_write_fp = fopen(write_file, "w");
+        test_write_fp = TEMP_FAILURE_RETRY_FOPEN(fopen(write_file, "w"));
     }
 }
 
 void c_end_disk_trace_6(void) {
     if (test_read_fp) {
-        fflush(test_read_fp);
-        fclose(test_read_fp);
+        TEMP_FAILURE_RETRY(fflush(test_read_fp));
+        TEMP_FAILURE_RETRY(fclose(test_read_fp));
         test_read_fp = NULL;
     }
     if (test_write_fp) {
-        fflush(test_write_fp);
-        fclose(test_write_fp);
+        TEMP_FAILURE_RETRY(fflush(test_write_fp));
+        TEMP_FAILURE_RETRY(fclose(test_write_fp));
         test_write_fp = NULL;
     }
 }
diff --git a/src/disk.h b/src/disk.h
index c9b87bb9..faadf60f 100644
--- a/src/disk.h
+++ b/src/disk.h
@@ -19,9 +19,10 @@
 
 #include "common.h"
 
-#define DSK_SIZE 143360
-#define NIB_SIZE 232960
-#define NI2_SIZE 223440
+#define ERR_IMAGE_NOT_EXPECTED_SIZE "disk image is not expected size"
+#define ERR_STAT_FAILED "disk image unreadable for stat"
+#define ERR_CANNOT_OPEN "could not open disk image"
+#define ERR_MMAP_FAILED "disk image unreadable for mmap"
 
 #define NUM_TRACKS 35
 #define NUM_SECTORS 16
@@ -30,6 +31,10 @@
 #define NIB_TRACK_SIZE 0x1A00 // NIB format
 #define NI2_TRACK_SIZE 0x18F0 // NI2 format
 
+#define DSK_SIZE (NUM_TRACKS * DSK_TRACK_SIZE) // 143360
+#define NIB_SIZE (NUM_TRACKS * NIB_TRACK_SIZE) // 232960
+#define NI2_SIZE (NUM_TRACKS * NI2_TRACK_SIZE) // 223440
+
 #define PHASE_BYTES (NIB_TRACK_SIZE/2)
 #define NIB_SEC_SIZE (NIB_TRACK_SIZE/NUM_SECTORS)
 
@@ -74,10 +79,17 @@ typedef struct drive_t {
 
 extern drive_t disk6;
 
-void c_init_6(void);
-const char *c_new_diskette_6(int drive, const char * const file_name, int force);
-const char *c_eject_6(int drive);
-void disk_io_initialize(unsigned int slot);
+// initialize emulated 5.25 Disk ][ module
+extern void disk6_init(void);
+
+// insert 5.25 disk image file
+extern const char *disk6_insert(int drive, const char * const file_name, int force);
+
+// eject 5.25 disk image file
+extern const char *disk6_eject(int drive);
+
+// flush all I/O
+extern void disk6_flush(int drive);
 
 #if DISK_TRACING
 void c_toggle_disk_trace_6(const char *read_file, const char *write_file);
diff --git a/src/interface.c b/src/interface.c
index b133b810..f78f4336 100644
--- a/src/interface.c
+++ b/src/interface.c
@@ -344,7 +344,7 @@ static char zlibmenu[ZLIB_SUBMENU_H][ZLIB_SUBMENU_W+1] =
     "||||||||||||||||||||||||||||||||||||||||" };
 
 static void _eject_disk(int drive) {
-    const char *err_str = c_eject_6(drive);
+    const char *err_str = disk6_eject(drive);
     if (err_str) {
         int ch = -1;
         snprintf(&zlibmenu[4][2], 37, "%s", err_str);
@@ -588,7 +588,7 @@ void c_interface_select_diskette( int drive )
                     /* reopen disk, forcing write enabled */
                     if (toupper(ch) == 'W')
                     {
-                        const char *err_str = c_new_diskette_6(drive, temp, 0);
+                        const char *err_str = disk6_insert(drive, temp, /*readonly:*/0);
                         if (err_str)
                         {
                             int ch = -1;
@@ -656,7 +656,7 @@ void c_interface_select_diskette( int drive )
                 _eject_disk(drive);
                 c_interface_print_screen( screen );
 
-                const char *err_str = c_new_diskette_6(drive, temp, (toupper(ch) != 'W'));
+                const char *err_str = disk6_insert(drive, temp, /*readonly:*/(toupper(ch) != 'W'));
                 if (err_str)
                 {
                     int ch = -1;
@@ -1283,9 +1283,9 @@ void c_interface_parameters()
                     if (ch == 'Y')
                     {
                         save_settings();
-                        c_eject_6( 0 );
+                        disk6_eject(0);
                         c_interface_print_screen( screen );
-                        c_eject_6( 1 );
+                        disk6_eject(1);
                         c_interface_print_screen( screen );
 #ifdef __linux__
                         LOG("Back to Linux, w00t!\n");
diff --git a/src/test/testcommon.c b/src/test/testcommon.c
index e7a0344c..34e3b00f 100644
--- a/src/test/testcommon.c
+++ b/src/test/testcommon.c
@@ -147,10 +147,10 @@ int test_setup_boot_disk(const char *fileName, int readonly) {
 #else
     asprintf(&disk, "%s/disks/%s", data_dir, fileName);
 #endif
-    if (c_new_diskette_6(0, disk, readonly)) {
+    if (disk6_insert(0, disk, readonly)) {
         int len = strlen(disk);
         disk[len-3] = '\0'; // try again without '.gz' extension
-        err = (c_new_diskette_6(0, disk, readonly) != NULL);
+        err = (disk6_insert(0, disk, readonly) != NULL);
     }
     FREE(disk);
     return err;
diff --git a/src/test/testdisk.c b/src/test/testdisk.c
index fd78ca06..8b0b0c38 100644
--- a/src/test/testdisk.c
+++ b/src/test/testdisk.c
@@ -59,7 +59,7 @@ TEST test_boot_disk_bytes() {
     BOOT_TO_DOS();
 
     c_end_disk_trace_6();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -107,7 +107,7 @@ TEST test_boot_disk_bytes_nib() {
     BOOT_TO_DOS();
 
     c_end_disk_trace_6();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -160,7 +160,7 @@ TEST test_boot_disk_bytes_po() {
     BOOT_TO_DOS();
 
     c_end_disk_trace_6();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -265,7 +265,7 @@ TEST test_savehello_dsk() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -291,7 +291,7 @@ TEST test_savehello_nib() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -317,7 +317,7 @@ TEST test_savehello_po() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -388,7 +388,7 @@ TEST test_disk_bytes_savehello_dsk() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_DSK, 1);
@@ -416,7 +416,7 @@ TEST test_disk_bytes_savehello_dsk() {
         ASSERT(strcmp(mdstr0, EXPECTED_BSAVE_DSK_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -482,7 +482,7 @@ TEST test_disk_bytes_savehello_nib() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_NIB, 1);
@@ -510,7 +510,7 @@ TEST test_disk_bytes_savehello_nib() {
         ASSERT(strcmp(mdstr0, EXPECTED_BSAVE_NIB_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -581,7 +581,7 @@ TEST test_disk_bytes_savehello_po() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_PO, 1);
@@ -609,7 +609,7 @@ TEST test_disk_bytes_savehello_po() {
         ASSERT(strcmp(mdstr0, EXPECTED_BSAVE_PO_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -650,7 +650,7 @@ TEST test_outofspace_dsk() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_DSK, 1);
@@ -678,7 +678,7 @@ TEST test_outofspace_dsk() {
         ASSERT(strcmp(mdstr0, EXPECTED_OOS_DSK_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -705,7 +705,7 @@ TEST test_outofspace_nib() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_NIB, 1);
@@ -733,7 +733,7 @@ TEST test_outofspace_nib() {
         ASSERT(strcmp(mdstr0, EXPECTED_OOS_NIB_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -760,7 +760,7 @@ TEST test_outofspace_po() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     // Now verify actual disk bytes written to disk
     test_setup_boot_disk(BLANK_PO, 1);
@@ -788,7 +788,7 @@ TEST test_outofspace_po() {
         ASSERT(strcmp(mdstr0, EXPECTED_OOS_PO_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -913,7 +913,7 @@ TEST test_bload_trace_dsk() {
     unlink(disk);
     FREE(disk);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1031,7 +1031,7 @@ TEST test_bload_trace_nib() {
     unlink(disk);
     FREE(disk);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1154,7 +1154,7 @@ TEST test_bload_trace_po() {
     unlink(disk);
     FREE(disk);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1181,7 +1181,7 @@ TEST test_inithello_dsk() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1206,7 +1206,7 @@ TEST test_inithello_nib() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1231,7 +1231,7 @@ TEST test_inithello_po() {
     ASSERT(apple_ii_64k[0][WATCHPOINT_ADDR] == TEST_FINISHED);
     ASSERT_SHA(BOOT_SCREEN);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1266,7 +1266,7 @@ TEST test_data_stability_dsk() {
         ASSERT(strcmp(mdstr0, EXPECTED_STABILITY_DSK_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1301,7 +1301,7 @@ TEST test_data_stability_nib() {
         ASSERT(strcmp(mdstr0, EXPECTED_STABILITY_NIB_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1336,7 +1336,7 @@ TEST test_data_stability_po() {
         ASSERT(strcmp(mdstr0, EXPECTED_STABILITY_PO_SHA) == 0);
     } while(0);
 
-    c_eject_6(0);
+    disk6_eject(0);
 
     PASS();
 }
@@ -1392,7 +1392,7 @@ GREATEST_SUITE(test_suite_disk) {
     RUN_TESTp(test_data_stability_po);
 
     // ...
-    c_eject_6(0);
+    disk6_eject(0);
     pthread_mutex_unlock(&interface_mutex);
 }
 
diff --git a/src/test/testdisplay.c b/src/test/testdisplay.c
index 0c7b104c..440990ab 100644
--- a/src/test/testdisplay.c
+++ b/src/test/testdisplay.c
@@ -433,7 +433,7 @@ GREATEST_SUITE(test_suite_display) {
     RUN_TEST(test_80col_hires);
 
     // ...
-    c_eject_6(0);
+    disk6_eject(0);
     pthread_mutex_unlock(&interface_mutex);
 }
 
diff --git a/src/test/testtrace.c b/src/test/testtrace.c
index e9cc4c42..c4caa1f1 100644
--- a/src/test/testtrace.c
+++ b/src/test/testtrace.c
@@ -59,7 +59,7 @@ TEST test_boot_disk_cputrace() {
     BOOT_TO_DOS();
 
     cpu65_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -110,7 +110,7 @@ TEST test_cputrace_hello_dsk() {
     c_debugger_go();
 
     cpu65_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -160,7 +160,7 @@ TEST test_cputrace_hello_nib() {
     c_debugger_go();
 
     cpu65_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -210,7 +210,7 @@ TEST test_cputrace_hello_po() {
     c_debugger_go();
 
     cpu65_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -254,7 +254,7 @@ TEST test_boot_disk_vmtrace() {
     BOOT_TO_DOS();
 
     vm_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -302,7 +302,7 @@ TEST test_boot_disk_vmtrace_nib() {
     BOOT_TO_DOS();
 
     vm_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -350,7 +350,7 @@ TEST test_boot_disk_vmtrace_po() {
     BOOT_TO_DOS();
 
     vm_trace_end();
-    c_eject_6(0);
+    disk6_eject(0);
 
     do {
         uint8_t md[SHA_DIGEST_LENGTH];
@@ -408,7 +408,7 @@ GREATEST_SUITE(test_suite_trace) {
 #endif
 
     // ...
-    c_eject_6(0);
+    disk6_eject(0);
     pthread_mutex_unlock(&interface_mutex);
 }
 
diff --git a/src/test/testvm.c b/src/test/testvm.c
index 2cae9a82..1d60f79a 100644
--- a/src/test/testvm.c
+++ b/src/test/testvm.c
@@ -3452,7 +3452,7 @@ GREATEST_SUITE(test_suite_vm) {
     RUN_TESTp(test_check_cxrom, /*CXROM*/1);
 
     // ...
-    c_eject_6(0);
+    disk6_eject(0);
     pthread_mutex_unlock(&interface_mutex);
 }
 
diff --git a/src/vm.c b/src/vm.c
index b34b3145..3bb18ed3 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -1195,7 +1195,6 @@ static void _initialize_tables(void) {
 #ifdef AUDIO_ENABLED
     mb_io_initialize(4, 5); /* Mockingboard(s) and/or Phasor in slots 4 & 5 */
 #endif
-    disk_io_initialize(6); /* Put a Disk ][ Controller in slot 6 */
 }
 
 // ----------------------------------------------------------------------------
@@ -1205,7 +1204,7 @@ void vm_initialize(void) {
     _initialize_apple_ii_memory();
     _initialize_tables();
     vm_reinitializeAudio();
-    c_init_6();
+    disk6_init();
     _initialize_iie_switches();
     c_joystick_reset();
 }