diff --git a/Device/BlockDevice.cpp b/Device/BlockDevice.cpp
index 79f6e86..9418d89 100644
--- a/Device/BlockDevice.cpp
+++ b/Device/BlockDevice.cpp
@@ -27,6 +27,30 @@ using ProFUSE::Exception;
 using ProFUSE::POSIXException;
 
 
+unsigned BlockDevice::ImageType(MappedFile *f, unsigned defv)
+{
+#undef __METHOD__
+#define __METHOD__ "BlockDevice::ImageType"
+    
+    
+    if (UniversalDiskImage::Validate(f, std::nothrow))
+        return '2IMG';
+    
+    if (DiskCopy42Image::Validate(f, std::nothrow))
+        return 'DC42';
+    
+    if (SDKImage::Validate(f, std::nothrow))
+        return 'SDK_';
+    
+    if (ProDOSOrderDiskImage::Validate(f, std::nothrow))
+        return 'PO__';
+    
+    if (DOSOrderDiskImage::Validate(f, std::nothrow))
+        return 'DO__';    
+    
+    return defv;
+    
+}
 
 unsigned BlockDevice::ImageType(const char *type, unsigned defv)
 {
@@ -96,27 +120,20 @@ BlockDevicePointer BlockDevice::Open(const char *name, File::FileFlags flags, un
     {
         throw POSIXException(__METHOD__ ": stat error", errno);       
     }
+
+    // /dev/xxx ignore the type.
+    if (S_ISBLK(st.st_mode))
+        return RawDevice::Open(name, flags);
+    
+    MappedFile file(name, flags);
+
     
     if (!imageType)
     {
-        // /dev/xxxx 
-        if (S_ISBLK(st.st_mode))
-            return RawDevice::Open(name, flags);
-        
-        
-        imageType = ImageType(name, 'PO__');
+        imageType = ImageType(&file, 'PO__');
     }
     
-    if (imageType == 'SDK_')
-    {
-        // opened by path name.
-        return SDKImage::Open(name);
-    }
-    // TODO -- if no image type, guess based on file size?
-    
-    MappedFile file(name, flags);
-    
-    
+
     switch (imageType)
     {
         case '2IMG':
@@ -133,6 +150,10 @@ BlockDevicePointer BlockDevice::Open(const char *name, File::FileFlags flags, un
             
         case 'DVX_':
             return DavexDiskImage::Open(&file);
+            
+        case 'SDK_':
+            return SDKImage::Open(name);
+                
                     
     }
     
diff --git a/Device/BlockDevice.h b/Device/BlockDevice.h
index c047912..4b31e71 100644
--- a/Device/BlockDevice.h
+++ b/Device/BlockDevice.h
@@ -12,6 +12,8 @@
 
 #include <File/File.h>
 
+class MappedFile;
+
 namespace Device {
 
     class BlockDevice : public ENABLE_SHARED_FROM_THIS(BlockDevice) {
@@ -20,7 +22,9 @@ public:
 
     // static methods.
     static unsigned ImageType(const char *type, unsigned defv = 0);
-    
+    static unsigned ImageType(MappedFile *, unsigned defv = 0);
+        
+        
     static BlockDevicePointer Open(const char *name, File::FileFlags flags, unsigned imageType = 0);
     static BlockDevicePointer Create(const char *fname, const char *vname, unsigned blocks, unsigned imageType = 0);
     
diff --git a/main.cpp b/main.cpp
index 137d0d3..62dff80 100644
--- a/main.cpp
+++ b/main.cpp
@@ -128,6 +128,7 @@ static void usage()
             "  --format=format   specify the disk image format. Valid values are:\n"
             "                    dc42  DiskCopy 4.2 Image\n"
             "                    davex Davex Disk Image\n"
+            "                    sdk   ShrinkIt Disk Image\n"
             "                    2img  Universal Disk Image\n"
             "                    do    DOS Order Disk Image\n"
             "                    po    ProDOS Order Disk Image (default)\n"