+- It appears to me that you have run into some sort of race
+ condition where two threads want to use the same Volume and they
+ were both given access. Normally that is no problem. However,
+ one thread wanted the particular Volume in drive 0, but it was
+ loaded into drive 1 so it decided to unload it from drive 1 and
+ then loaded it into drive 0, while the second thread went on
+ thinking that the Volume could be used in drive 1 not realizing
+ that in between time, it was loaded in drive 0.
+ I'll look at the code to see if there is some way we can avoid
+ this kind of problem. Probably the best solution is to make the
+ first thread simply start using the Volume in drive 1 rather than
+ transferring it to drive 0.
+- After pruning, check to see if the Volume retention period has
+ expired.