Volumn on disk not accessible

On my second hdd I have 3 partitions, one of which I created to use
with TrueCrypt. This is mounted as Z:

It's worked fine for months, but suddenly I cannot access it, either
with TrueCrypt, or even with XPs disk management program to reformat
it (I suspect it will even prevent me from deleting it)! In both cases
I get an error message saying that the volume in use by some other
program, but I cannot find out what. I've excluded Z: from the real
time scanner in Avast, and also excluded it from my Genie Timeline
Backup monitoring (it was never in there anyway). The XP disc
management program reports the volume as healthy but full (presumably
because it's encrypted with TrueCrypt.

How can I find out what's using this volume?

I have a backup of the data that's in this volume, so don't mind
"taking a hammer to it" if necessary.

Thanks, David