The inability to remove devices from ZFS Zpools has been one of the most annoying / inflexiable things about ZFS. I once read a blogpost about somenone who added a USB flash stick into the root ZFS pool, and now the USB stick became a perminent fixture of the machine! There was no simple way to fix this issue other than backup / network dump / rebuild machine / network restore.
Matthew Ahrens workedon changing the ZFS scrub code in SNV 94 (which made its way into the Fishwork’s Amber Road NAS Appliance), but this work will also lay the groundwork for “zpool evict” or the such. This is a great read on the internal working of how blockpointers are used within ZFS.