When a sector can't be written to for whatever reason, it's marked as pending, if it fails again on the next attempt then it will reallocate it to one of the spares. It has nothing to do with the slot, unless you were shaking the drive while in the slot while it was being written to...
The drive is bad, but it's your data so go nuts. Keep good backups, because if you need that drive to restore another one, cross your fingers, if another sector goes bad your parity is lost.