>> What is the brand and type of this SCSI disk?
>
>ahc0: target 0 synchronous at 20.0MHz, offset = 0xf
>ahc0: target 0 using tagged queuing
>sd0 at scsibus0 target 0 lun 0: <QUANTUM, XP34550S, LXY1> SCSI2 0/direct fixed

You should upgrade this driver to LXY4.  The drive may hang under heavy
tag load otherwise.

Another issue with the Atlas drives is that they will return QUEUE FULL
status if their write cache fills up even if you only have a 1 or 2
commands outstanding.  This may be confusing the NetBSD SCSI layer.

--
Justin