Strictly speaking, this is not a NetBSD kernel issue. However, I hope that someone more familiar with mpt(4) has come accross that MPT "feature" before:
One additional oddity I faced with Thursday's disc failure was that after physically replacing the failed disc with a spare, the SAS controller decided to assign a new pseudo target id to the new disc, skipping the old disc's 3 and using 8 for the replacement (still at PHY 3). Since the kernel assigns sd and, thus, dk numbers in the order of increasing SCSI target ids, this meant my RAIDframe components now became dk2, 6, 3, 4, 5 (not 2, 3, 4, 5, 6 as before). Not a big deal, but somewhat confusing at midnight. I assume that's intentional behaviour to the benefit of some commercial or GPL-licenced OS software. The renumbering was persistent accoss both a hard reset and a soft power cycle (I didn't try a hard power cycle). Is there a way of reverting to the old sequence or disabling that nonsense in the first place?