/netbackup/db/images/client/xxxxxx directories and .lck file

As you might have already guessed, they are lock files.  :)  We need them to prevent NetBackup from trying to do two things to one object.  (What object, you ask?  I don't know for sure if it's directory or image or something else...and I didn't have enough time to investigate further)  The files are associated with some locking that takes place and USUALLY disappear when their related images (they share most of the same filename, you'll find) expire.  Sometimes they don't.  That's probably a defect.

What I CAN tell you is that any "orphaned" lock files should eventually get cleaned up when their directories are aged out and get removed.  This happens when there are no more valid images in those directories.

I can also (perhaps unreasonably) speculate that there are plans to move away from this kind of behaviour, but I have no idea by what version something like that might get accomplished as that's well above my pay grade.

The bottom line is those .lck files are very crucial at the time they are created, and then later, much less important if they end up sticking around long after their associated images are gone. :)