USB Drive changing letter sometimes?

Whilst I haven't had any issues myself with changing drive letters I know several people who recommend Uwe Seiber's USB Drive Letter Manager.

(I have used other Uwe Seiber utilities.)

Hope this helps...

Just to give you a good feeling save your "Mounted Devices"

REG EXPORT HKLM\SYSTEM\MountedDevices E:\Backup\Mount_REG.reg (Just an example for the added path)
to reimport that *.reg type
REG IMPORT E:\Backup\Mount_REG.reg

In Mounted Devices sometimes you find a looooong list.
So just delete all that stuff by this command:
reg delete HKLM\SYSTEM\MountedDevices
Reboot and you have a clean list!

If you don't like clean lists type the above command:
REG IMPORT E:\Backup\Mount_REG.reg

It kinds of sounds like what I am hearing is that Windows assigns a unique ID (not drive letter) in the system volume file on USB along with label name- but not a drive letter. Then when drive is plugged in or assigned via disk management, a letter is assigned to that unique ID and stored in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices. Then when that device is plugged in later and if that unique ID is found in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices it will use that drive letter? Sounds like it is kind of table driven- which seems stupid.
That is not the way things worked on mainframe systems, disk ID were stored on system volume file and stayed with it no matter what computer you would mount that disk. If you had duplicates, you would get error messages.
But I still may not be getting it and have been retired now since 12/1/2003, things change. Also, it easy enough to just change the drive letter if needed. Still seems PC are making it more complicated that the mainframe world did where you put an ID on the disk and it sticks with it, if you are stupid enough to name two the same and try and have them mounted- you will get an error.

diverjer said:

It kinds of sounds like what I am hearing is that Windows assigns a unique ID (not drive letter) in the system volume file on USB along with label name- but not a drive letter. Then when drive is plugged in or assigned via disk management, a letter is assigned to that unique ID and stored in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices. Then when that device is plugged in later and if that unique ID is found in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices it will use that drive letter? Sounds like it is kind of table driven- which seems stupid.

My understanding is that when Windows enumerates the device it assigns an Instance ID and stores a record of it in the registry. I believe drive letter assignment is secondary to the initial enumeration. However, IME, enumeration is not consistent.

Have a look at this screenshot of USB enumeration details as seen by Nir Sofer's USBDeview:


The two highlighted entries are the same USB device - I only have one of these. The ONLY difference is that I attached them on different days... yet the enumeration is different. You can see by the Serial Number and Instance ID columns that it's the same device... just enumerated differently.

Even weirder, you can see see in the Connected column that the device is connected... yet apparently has no drive letter, even though File Manager shows it as drive E: and I'm using it quite happily as a mass storage device.

It's my belief that this is another symptom of a long-standing bug with how Windows enumerates USB devices and fails to record details accurately and/or consistently (and never removes the records from the registry automatically)... hence the impossibility of guaranteeing the same drive letter each time the device is connected.

Hope this helps...

I have never lost my only manual assignment for an USB stick that I consider "special", and from time to time I backup its files to other two (last time it was for some motherboard drivers that failed when tried, although I consider them potentially useful). This backupping is the actual reason why I assign the last permanent letter plus 2 to this drive (it allows plugging it and the backup one in any order at the same time, and both get the intended letter and the backup profile works, all w/o doing manual assignments). The last permanent letter plus 1 is the one that all other USB sticks catch, as I never connect more than one.

If I decided to get covered against having to plug 2 sticks "w/o pre-assigned letter", I would assign the last permanent letter plus 3 to my special pendrive, plus 4 if it's 3 sticks etc

And in theory I could have more than one "special" USB drive. For instance:

E: is the last permanent letter, no external drive will catch it or a lower one because they belong to internal drives. The computer has 4 "own" USB drives but we want to allow up to 3 "foreign" ones (w/o preassigned letter).

Solution (the simplest one, but requires 7 free USB sockets): first plug 3 "foreing" drives in whatever order, they will get the letters F: G: H: in plugging order (Windows will memorize this assignment, but we don't care about it; after we finish, these letters could be "stolen" by other "foreign" drives). Then plug the 4 "own" drives, that will get the following letters I: J: K: L: in plugging order. Windows will memorize these assignments, this is what we want. Afterwards we unplug all drives in any order. Provided that we never attach more than 3 "foreign" drives at the same time, no drive will steal any of the I: J: K: L: letters.

All other solutions along with updates and corrections (for instance we replace one of the I: J: K: L: drives) will require assigning letters manually.

As said I have never seen Windows failing in recalling that my only "cared about" drive has the last permanent letter plus 2, but the example above is more complicated. I use this pendrive quite frequently, Idk if this matters. I don't know either if Windows has "worse memory" for late letters like R: S: T: U: .

Thanks for that explanation, it does help me understand a little better what is going on in the system software. It's not how I would do it, but when dealing with Personal Computers and all the clutter of hardware/software combinations- it has to be difficult to have any standards.

dalchina said:

Intriguing.. loooks like it was connected by different means although on the same hub on the two occasions?

It's a 2TB SSD in an iDSONIX enclosure connected via USB:


It can be seen in both File Manager and Disk Management, complete with drive letter:


The laptop I took the screenshot from doesn't have USB C ports, only USB A... so that's not a factor. It's just how it's recorded in the registry that differs.

JLArranz said:

I don't know either if Windows has "worse memory" for late letters like R: S: T: U: .

Not in my experience. However, it has a long-standing USB enumeration bug that manifests itself after repeated insertions of USB devices. The symptoms are Windows failing to recognise a USB device (or failing to assign a drive letter) that works perfectly in other Windows machines. When this happens I use Nir Sofer's USBDeview to fix the issue by uninstalling all non-connected USB devices. This removes the enumeration entries stored in the following location:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\USB\