My old HDD was having some issues and needed to be swapped, I cloned C: to a new drive, removed the old drive and it wouldn’t boot up as C:\, only D:\…so when I logged in it basically just sat there looking for the C:\ which was actually assigned as D:\ …

Modifying the boot settings didn’t do anything, using diskpart to attempt to change the drive letter didn’t work… fail fail fail.  Eventually, I realized that things in Vista / Windows 7 may have changed a bit bootloader wise.  I booted up my computer, CTRL+ALT+DEL’d to get a task manager, and ran REGEDIT.

There it was, “HKLM\SYSTEM\MountedDevices”.

This is basically a list of drives and drive letters that are assigned to those drives.

I deleted all of the entries, rebooted, and voila! Back in business!

Here’s what happened:

In Vista and Windows 7, if you clone your active / main / boot drive (via R-Drive Drive Image or the likes), and Windows sees it with the original drive still plugged in and active, it will set a fixed drive letter entry in your “Mounted Disks” registry folder.

When you remove the first drive and put the new one on the same I/O cable, it will boot, but it will either reference the already created drive letter in the registry, or will not assign C:\ to the “new” drive’s serial number. Even though you cloned C:\ to an new disk, and removed the old C:\ physically, the letter will still be reserved in the registry, and the next (in my case anyways) available letter is D:\.

Hope this helps someone out there!