Solved Do W7 assigned Drive Letters really help or hurt?

[Mike Lynch]

I'm lost regarding them.
I believe Windows assigns some temporally.
I believe I assign some permanently, i.e. (the Drive letter is written to the Drive somehow.)

I'd like to know how it's really done AND how to undue it when desired.

Please refer me to an Answer or Explain it for me.

If I wanted to, is there a way to restore my PC to the state where it only had the C: Drive assigned, and all the other potential Drives were available to be assigned a Drive Letter? (A, B, D THRU Z) C already being assigned.

 

[MisterEd]

Only the C: drive assignment really matters. Also, if you install programs in another partition then you should not change that drive letter assignment either. All the other driver letter assignments can be changed if you want. The easiest way to change a drive letter is with Disk Management. Besides external drives you can have virtual CD/DVD drives and network drives. Sometimes it doesn't matter what the drive letter is so you can change it. However, you might want to keep the drive letter always the same. An example is drive used for backups because the backup program expects to backup to a certain drive letter.

 

[dg1261]

Windows stores drive letter assignments in the [HKLM\System\MountedDevices] registry key. Open up that registry key and you'll see a bunch of values like "\DosDevices\C:" and more. That is where Windows keeps track of what device partitions it's seen and what drive letters have been assigned to them.

Windows surveys attached drive partitions and derives a "partition signature" for each. That signature and its assigned drive letter, if any, is recorded in the registry. The signature is how Windows knows if it has seen that partition before. If the device (such as a flash drive or USB external drive) is removed and later reinserted, Windows will recognize if it had previously seen that partition signature, and will give it the same drive letter if it can.

OTOH, when a previously assigned device is no longer in the system Windows will consider the now dormant drive letter up for grabs and available for reassignment to a different device. If that happens and the drive letter gets reassigned to a different device, when the previous device is reinserted Windows will not remember it had previously assigned a drive letter, and the next available drive letter will be assigned to the reintroduced device.

For illustration, say you insert flash drive 1 and flash drive 2 at the same time. Let's say Windows assigns "E:" and "F:" as their drive letters. Now you remove both flash drives. If you subsequently reinsert only flash drive 2, it will still be "F:" and there will be no "E:".

Now let's say you insert flash drive 3 (while flash drive 1 is still out). Windows will reuse the "E:" letter because it's not currently in use. If you then insert flash drive 1 again, it will have to get a new drive letter because "E:" is currently in use.

Being able to keep the same drive letter can be useful -- for command scripts or batch files, for instance. Accordingly, if you want a removable device to retain the same drive letter time after time, it's wise to manually change its drive letter (via Disk Management) to something higher in the alphabet (such as "N:" or "U:", for example). Even when the removable device is out of the system and its drive letter is technically up for grabs, it's unlikely Windows will get far enough through the alphabet to reassign that letter to something else, and your scripts and batch files won't get derailed.

If you want to manually force Windows to forget previous drive letter assignments, simply delete the corresponding "\DOSDevices\x:" entry from the registry. Deleted drive letters will be available for fresh assignment to newly detected devices.

 

[Mike Lynch]

dg1261

I need another paragraph rather than just assuming and getting myself into trouble.

If I never assign any Drive Letters to any External Devices, how should life proceed?

Thanks for bearing with me on this.

Mike

 

[dg1261]

When a drive letter has been assigned, there's always a pair of entries for any device/partition. In the [MountedDevices] key you'll find the "\DosDevicesC: =" value followed by a long string of bytes, and elsewhere you'll find that same string associated with another string of bytes. The latter is a "signature", a sort of handle derived from hardware identifiers. The other is the drive letter the system is associating with that signature.

If you have, for example, a flash drive that you've never plugged into that system, of course nothing about it is recorded in the registry yet. When you plug it in and a drive letter is assigned, both key values are generated and stored in [MountedDevices]. If the flash drive later loses its drive letter (either because you manually removed the drive letter or something else stole the drive letter), the remaining "signature" part of the pair remains in the registry.

That is why you'll often see a whole horde of mysterious, unidentifiable entries in [MountedDevices] -- most of them are leftovers from every flash drive or external drive that's ever been plugged in. (FTR, keep in mind that USB drives may get different signatures depending on which USB port you use, so a single flash drive may create multiple key values if it's switched from port to port.)

If you really care, you can prune those excess entries (any entry that doesn't have a corresponding drive letter entry), but it has to be done by manually editing the registry. I know of no Windows tool that will scan the registry and remove dormant or obsolete entries.

If you accidentally delete an active entry, note Windows will regenerate it upon reboot. In fact, even if you delete *all* entries from [MountedDevices], the next time Windows boots it will rebuild just the active entries (the devices and partitions Windows sees on its bootup survey). Windows will remember no drive letters on first reboot, so the key values will be regenerated following the standard sequence of assigning drive letters that Windows has followed since DOS days. This is a quick way to make Windows forget every drive letter assignment it previously had (and every obsolete entry, as well), thereby starting over with fresh drive letter assignments upon reboot.

(Fortunately, under that old DOS sequence, the OS assigns "C:" to its own partition, so even if you delete C: from the registry the same letter will be regenerated upon reboot, and Windows will not lose track of itself. Don't try this if your OS "Boot" partition is other than C:, though.)

Incidentally, [MountedDevices] is the reason for the rule of thumb advising one to remove the old OS disk when booting a clone for the first time. If the original disk is still installed the first time a clone boots, the clone's registry (copied from the original disk) may still have C: assigned to the original signature, and if the original is still in the system the clone will have to assign itself the next available drive letter instead of C:, and that change will evermore be locked into its registry. Then you have what I call a "schizophrenic system", where the cloned OS is not C: but so many .ini files and other registry entries looking for C: will look to the old disk for configuration information.

In contrast, if the old disk is removed from the system before the clone's first boot, the clone's registry will see an invalid signature for C: so that letter will be available for reassignment. Since the cloned registry doesn't yet have a letter for its own partition, it will reassign C: to itself, and all is well. If the old disk is subsequently reinserted into the system, its C: letter will already have been reclaimed by then, so the old OS partition cannot be C: and will get the next available drive letter.