The quote Jack offered is relevant to PRIMARY pools, but not to COPY pools,
I think.

TSM reclaims COPY (off-site) storage pools by using the drives and tapes
available to the PRIMARY (on-site) storage pool.

It can do this because its massive database knows where in the PRIMARY pool
to find a copy of each file on the COPYpool tape it's reclaiming, so it can
make a duplicate of an off-site copypool tape even though it can't
physically mount that offsite copypool tape.

Jon, surely your onsite pool has two or more drives, right?
Maybe reclamation didn't start because it's a relatively low-priority
process (look up "preempt" in the Admin guide), and something more important
(like backup db) was running.