diff --git a/modules/diskdump/lib/diskdump.js b/modules/diskdump/lib/diskdump.js
index a8698f30f..e10079266 100644
--- a/modules/diskdump/lib/diskdump.js
+++ b/modules/diskdump/lib/diskdump.js
@@ -2142,14 +2142,18 @@ DiskDump.prototype.convertToJSON = function()
         var cbDiskData = this.bufDisk.length;
 
         if (cbDiskData >= 3000000) {        // arbitrary threshold between diskette image sizes and hard disk image sizes
-            /*
-             * In this case, the first sector should be an MBR; find the active partition entry,
-             * then read the LBA of the first partition sector to calculate the boot sector offset.
-             */
-            for (var offEntry = 0x1BE; offEntry <= 0x1EE; offEntry += 0x10) {
-                if (this.bufDisk.readUInt8(offEntry) >= 0x80) {
-                    offBootSector = this.bufDisk.readUInt16LE(offEntry + 0x08) * cbSector;
-                    break;
+            var wSig = this.bufDisk.readUInt16LE(0x1FE);
+            if (wSig == 0xAA55) {
+                /*
+                 * In this case, the first sector should be an MBR; find the active partition entry,
+                 * then read the LBA of the first partition sector to calculate the boot sector offset.
+                 */
+                for (var offEntry = 0x1BE; offEntry <= 0x1EE; offEntry += 0x10) {
+                    if (this.bufDisk.readUInt8(offEntry) >= 0x80) {
+                        offBootSector = this.bufDisk.readUInt32LE(offEntry + 0x08) * cbSector;
+                        cbDiskData = this.bufDisk.readUInt32LE(offEntry + 0x0C) * cbSector;
+                        break;
+                    }
                 }
             }
             /*
diff --git a/modules/shared/lib/diskapi.js b/modules/shared/lib/diskapi.js
index 809513046..81b38ab15 100644
--- a/modules/shared/lib/diskapi.js
+++ b/modules/shared/lib/diskapi.js
@@ -83,7 +83,11 @@ DiskAPI.DISKETTE_FORMATS = {
     737280:  [80,2,9],          // media type 0xF9: 80 cylinders, 2 heads (double-sided),  9 sectors/track, (1440 total sectors x 512 bytes/sector ==  737280)
     1228800: [80,2,15],         // media type 0xF9: 80 cylinders, 2 heads (double-sided), 15 sectors/track, (2400 total sectors x 512 bytes/sector == 1228800)
     1474560: [80,2,18],         // media type 0xF0: 80 cylinders, 2 heads (double-sided), 18 sectors/track, (2880 total sectors x 512 bytes/sector == 1474560)
-    2949120: [80,2,36]          // media type 0xF0: 80 cylinders, 2 heads (double-sided), 36 sectors/track, (5760 total sectors x 512 bytes/sector == 2949120)
+    2949120: [80,2,36],         // media type 0xF0: 80 cylinders, 2 heads (double-sided), 36 sectors/track, (5760 total sectors x 512 bytes/sector == 2949120)
+    /*
+     * The following are common early hard disk sizes, which we explicitly map to CHS values, since the BPB can mislead us when attempting to calculate total cylinders
+     */
+    21368320:[615,4,17]         // PC AT 20Mb hard disk (type 2)
 };
 
 DiskAPI.MBR = {