Since this threads been resurrected I'll note that I did some more testing as it relates to the original OP's post, I won't bore you with the details, but...
@
kovidgoyal - when you detect something is locked, it should be possible to find out what process (program) is holding the lock (a'la what Unlocker and similar gadgets do), perhaps you could include that information in the locked file/folder message.
Knowing which program is holding the lock would assist users to do whatever is needed to unlock the folder or file.
There maybe a situation where calibre itself is holding the lock, that's what seems to happen to X2 in some corner/edge/boundary conditions which can be most often triggered by playing videos in its preview panel**
X2 prevents 'itself' from deleting a folder via its own folder lock, if I remove the lock with Unlocker, the delete can proceed. Otherwise I can be left with empty folders lying around that I swear I'd deleted.
BR
** I have no idea why anyone would want to do that, but the only way to stop it it to remove WMP - but that's a problem for another place