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!