I use a very similar system--the network hard drive came with its own automated backup program. The suspenders backing up the belt is the xcopy batch file, which produces an ordinary DOS file folder tree, unlike the backup system's compressed archive files which can be opened only by the backup program.

Similar setup, too: XP workstation, 2000 server. I experience the locked-file problem from time to time too, but with a server you expect that: All sorts of autonomous processes running that might access files, especially the Web server accessing the Web files that I'm trying to work on. In that case, if a file gets locked because some software I'm working on opened a file then crashed before closing, I have to restart the Web server. File locks in all OSes can be a bitch that way, and unfortunately Windows doesn't seem to provide an easy way to administratively cancel the lock. Locks expire with time, usually, but if you can't wait, sometimes all you can do is reboot the PC.

You mentioned using tiny files to tag the date/time of last backup. If you adopt the use of xcopy in batch files, here's a tip for automating that too:

date /t > \\VAULT\Robert\Mirror\C\log.txt
time /t >> \\VAULT\Robert\Mirror\C\log.txt

The first line overwrites the existing log.txt with a line containing the date, and the second line appends the time to the new file. The filename doesn't reveal the timestamp (I don't think DOS batch commands are up to that job), but the internal record is an ironclad timestamp because these lines run after the copying.

I don't figure this has been all that helpful, 'fraid I don't have a silver-bullet answer. Good luck...