If we're talking about inside Windows, then the drive letters are merely
relational to Windows; they're not written to the disk itself.
So you can do the following, which I've often done.
1. Plug in a USB external disk, and have it assigned a letter by Win7. That
might be H.
2. Clone the system C disk to it.
3. Take that disk and replace the original cloned one.
4. Boot. And it will have been assigned C.
That's not what I think the OP meant, but who knows?
What I think the OP meant is what I (think I) wrote about, and here's a
concrete example to clarify:
Let's say I plug in a drive, and Windows calls it drive F:. Now I
generate a backup script using Macrium.
Later I run that script, and Macrium expects (requires) the drive to be
drive F:. OK, except that today I happened to have plugged in a USB
stick, and the BU drive is now G:, so the backup task aborts - and
letter F: is not available to fix it. Unless I unplug the USB stick,
but if I happen to be running the script overnight while I sleep, this
option is not really viable...PITA.
Here's how I deal with that. When I first plug in the drive (or later,
after the above failure) I assign a high letter to the drive, such as
X:, and recreate the backup script.
Next time I plug the drive in, Windows remembers the X: and Macrium
finds the drive. No more problems, unless I assign X: to a second
device - but I'm not that dumb.