diff --git a/driver.c b/driver.c
index 6ee63e8..8aa636a 100644
--- a/driver.c
+++ b/driver.c
@@ -9,6 +9,7 @@
 #include "readtcp.h"
 #include "tcpconnection.h"
 #include "asmglue.h"
+#include "mounturl.h"
 #include "version.h"
 
 #define BLOCK_SIZE 512
@@ -221,23 +222,33 @@ static enum NetDiskError CheckTwoImg(Session *sess) {
 
 static Word DoMountURL(struct GSOSDP *dp) {
     enum NetDiskError err;
+    struct MountURLRec *mountURLRec = (struct MountURLRec *)dp->controlListPtr;
+
+    if (mountURLRec->byteCount != sizeof(struct MountURLRec)
+        || mountURLRec->byteCount != dp->requestCount)
+    {
+        dp->transferCount = 0;
+        return drvrBadParm;
+    }
 
     if (dp->dibPointer->extendedDIBPtr != NULL) {
         dp->transferCount = 0;
+        mountURLRec->result = DISK_ALREADY_MOUNTED;
         return drvrBusy;
     }
 
     Session *sess = calloc(sizeof(*sess), 1);
     if (sess == NULL) {
         dp->transferCount = 0;
+        mountURLRec->result = OUT_OF_MEMORY;
         return drvrNoResrc;
     }
     
-    err = SetURL(sess, (char*)dp->controlListPtr, TRUE, FALSE);
+    err = SetURL(sess, mountURLRec->url, TRUE, FALSE);
     if (err != OPERATION_SUCCESSFUL) {
-        // TODO arrange for more detailed error reporting
         EndNetDiskSession(sess);
         dp->transferCount = 0;
+        mountURLRec->result = err;
         return drvrIOError;
     }
     
@@ -246,6 +257,7 @@ static Word DoMountURL(struct GSOSDP *dp) {
         // TODO arrange for more detailed error reporting
         EndNetDiskSession(sess);
         dp->transferCount = 0;
+        mountURLRec->result = err;
         return drvrIOError;
     }
     
@@ -258,8 +270,8 @@ static Word DoMountURL(struct GSOSDP *dp) {
     err = CheckTwoImg(sess);
     if (err != OPERATION_SUCCESSFUL) {
         EndNetDiskSession(sess);
-        // TODO better error
         dp->transferCount = 0;
+        mountURLRec->result = err;
         return drvrIOError;
     }
     
@@ -267,6 +279,7 @@ static Word DoMountURL(struct GSOSDP *dp) {
     
     //TODO report disk switch
     
+    mountURLRec->result = OPERATION_SUCCESSFUL;
     return 0;
 }
 
diff --git a/http.c b/http.c
index 83c86e9..1cbaea3 100644
--- a/http.c
+++ b/http.c
@@ -206,7 +206,11 @@ netRetry:
         }
               
     default:
-        result = UNSUPPORTED_RESPONSE;
+        if (sess->responseCode < 400 || sess->responseCode > 599) {
+            result = UNSUPPORTED_RESPONSE;
+        } else {
+            result = sess->responseCode;
+        }
         goto errorReturn;
     }
 
diff --git a/mounturl.c b/mounturl.c
index d8da55e..c799a4c 100644
--- a/mounturl.c
+++ b/mounturl.c
@@ -1,19 +1,27 @@
 #include <stdlib.h>
 #include <string.h>
+#include <stdio.h>
 #include <gsos.h>
-
-#define MountURL 0x8080
+#include "mounturl.h"
 
 int main(int argc, char **argv) {
+    struct MountURLRec mountURLRec;
     DAccessRecGS controlRec = {5};
     
     if (argc < 3)
         return;
     
+    mountURLRec.byteCount = sizeof(mountURLRec);
+    mountURLRec.url = argv[2];
+    
     controlRec.devNum = strtoul(argv[1], NULL, 0);
     controlRec.code = MountURL;
-    controlRec.list = argv[2];
-    controlRec.requestCount = strlen(argv[2]) + 1;
+    controlRec.list = (Pointer)&mountURLRec;
+    controlRec.requestCount = sizeof(mountURLRec);
     
     DControl(&controlRec);
+    
+    if (mountURLRec.result != OPERATION_SUCCESSFUL) {
+        fprintf(stderr, "MountURL error %u\n", mountURLRec.result);
+    }
 }
diff --git a/mounturl.h b/mounturl.h
new file mode 100644
index 0000000..3a3feaa
--- /dev/null
+++ b/mounturl.h
@@ -0,0 +1,15 @@
+#ifndef MOUNTURL_H
+#define MOUNTURL_H
+
+#include "netdiskerror.h"
+
+/* Custom DControl code and request code for MountURL operation */
+#define MountURL 0x8080
+
+struct MountURLRec {
+    Word byteCount;
+    enum NetDiskError result; /* output value */
+    char *url; /* C-string; will be modified */
+};
+
+#endif
diff --git a/netdiskerror.h b/netdiskerror.h
index 282b7c0..54315d3 100644
--- a/netdiskerror.h
+++ b/netdiskerror.h
@@ -3,9 +3,12 @@
 
 enum NetDiskError {
     OPERATION_SUCCESSFUL = 0,
+    
+    DISK_ALREADY_MOUNTED = 100,
+    OUT_OF_MEMORY,
 
     /* SetURL errors */
-    URL_TOO_LONG = 100,
+    URL_TOO_LONG = 200,
     INVALID_CHARACTER_IN_URL,
     BAD_URL_SYNTAX,
     UNSUPPORTED_URL_SCHEME,
@@ -16,10 +19,9 @@ enum NetDiskError {
     IPV6_NOT_SUPPORTED,
     HOSTNAME_TOO_LONG,
     NAME_LOOKUP_FAILED,
-    OUT_OF_MEMORY,
     
     /* StartTCPConnection and DoHTTPRequest errors */
-    NETWORK_ERROR = 200,
+    NETWORK_ERROR = 300,
     NO_RESPONSE,
     INVALID_RESPONSE,
     EXCESSIVE_REDIRECTS,
@@ -29,8 +31,11 @@ enum NetDiskError {
     NOT_DESIRED_CONTENT,
     DIFFERENT_LENGTH, /* Might be considered successful later */
     
+    /* Error values of 4xx and 5xx mean we got the corresponding HTTP error */
+    HTTP_ERROR = 400,
+    
     /* File format errors */
-    UNSUPPORTED_2IMG_FILE = 300,
+    UNSUPPORTED_2IMG_FILE = 600,
     
 };