Hi Mike,
Many thanks for your reply. I'll try allocating Z: to the 500GB drive (instead of G:)as per your suggestion. Should I do the same thing to the other PG drive independently and allocate Z: instead of F: ?

My object is to retain the PG drive as a clone backup and to keep each one updated. The two drives will not be plugged in at the same time so am I right in assuming that the same drive label of Z: can be reserved for both?

I have no BIAB links or shortcut from my C:drive I just run BIAB/RB direct from the USB drive.

As I'm using up 4 of my 6 USB ports for other items (printers, wireless keyboard, audio interface etc.) I prefer to unplug any USB drives and only plug them in when needed to one of the remaining 2 ports. That may be part of my problem? Maybe there has been an odd occasion when I switched off the computer before unplugging the BIAB drive.

BIAB 2022 PLUSPAK (ext HD), 49PAKs, STYLESPAKs 1 - 6, Win 7 64bit PC, REAPER, Ozone, Adobe Audition CS6, EZ Drummer3 + EZ Keys,Soundcraft MTK22, Behringer X Touch Universal Controller,, SE Gemini/Rode NT2/Neumann/Shure mics