Drive letter changes do not stick


I have a removable hard drive connected by USB for which I have changed the drive letter from the automatically assigned 'K' to my choice of 'T'. In the past the assignment has occasionally defaulted to 'K', but recently I have found that my own assignment will not stick from one session to another. I use the drive for backup, and plug it in only at the end of the day for my backup session using a program in which my 'T' drive is named, so currently I have to change the letter from 'K' to 'T' every time I need to do the backup. I have WindowsXP SP3. Are there any suggestions as to how to deal with the problem? As I say, it is only quite recent.
Hi Rio, welcome to NST
Have you changed the backup disk recently. ?
If you have a disk letter assigned, it's associated with the unique identifier serial number burned into the device in a registry entry at HKLM/System/Local Devices.
New devices plugged in, for which a permanent letter has not been allocated, will be given the first unused letter available. Presumably you have C: to J: in use already, so it's picking K:
If your assignment of T: is not sticking, I'm wondering whether T: is already assigned to a different UID.
Have a look at the registry keys and see if T: is allocated, then plug in your ext HDD and see if it comes up as K: whether the identifier is different.
If T: exists and is not your ext HDD, try deleting that particular registry entry, then use disk management to assign it and see if it sticks.
Thanks for an informative response. I have just plugged in the HDD and it comes up 'T' as desired - no problem on this occasion. I have three USB HDDs, one with an integral switch, and two without, one of which is my 1 Terabyte backup (hence the 'T'). The other HDD without a switch is in constant use, and the HDD with a switch is too small for my backup needs. It's evident that right now the registry entry is unchanged from when I last switched off the computer, but if and when the drive reverts to 'K' I'll look to see if the 'T' entry has disappeared. On the daily occasions when I need the backup drive it's no great task to correct the drive letter if it has gone astray, but if the registry entry is being deleted by something in my system I should like to find the cause. Incidentally, my plan is to have a second backup HDD which I would alternate with the current one so that I can always have one off-site, and your reference to the UID makes me see that the alternate HDD would need a different letter with a corresponding second setting in the backup program. I use Cobian Backup, and perhaps the easiest solution is to have a basket of tasks each for a different drive letter - 'T', 'K', and the new one - Cobian Backup will pass over any task that addresses a missing location and will then perform the backup on whichever is currently valid. But I should still like to know what causes the trouble!