Updated RK03 disk information

disks/dec/rk03/README.md

@@ -13,3 +13,20 @@ RK03 disks are single-platter cartridges with 203 tracks per side, 12 sectors pe
 
 * [RT-11 v4.0](rtl11v4/) [[source](http://skn.noip.me/pdp11/rk1.dsk)]
 * [XXDP+ Diagnostics](xxdp/) [[source](http://skn.noip.me/pdp11/rk2.dsk)]
+
+---
+
+While the geometry of an RK03 disk implies that the total image size should be 2,494,464 bytes, not all the
+disks we've archived started out that way.  For example, the RT-11 v4.0 disk image was originally 1,454,592
+bytes long.  Presumably, when it was used by other emulators, they assumed zeros for the missing sectors.
+
+Since the PCjs [DiskDump](/modules/diskdump/) utility relies on exact file sizes to match disk images to
+supported geometries, I padded the disk image:
+
+	dd if=/dev/zero bs=1 count=1039872 >> RK03-RT11-V40.dsk
+
+After appending an additional 1,039,872 bytes to the original 1,454,592, DiskDump was happy to process the
+image:
+
+	diskdump --disk=RK03-RT11-V40.dsk --format=json --output=RK03-RT11-V40.json
+	2494464-byte disk image saved to RK03-RT11-V40.json