Adding Adaptec SCSI board - kernel trap!

I have an old MS-DOS system (actually an Amiga bridge board) that
has died. Its hard disk is still intact and there are some files
I'd very much like to recover from it. (No, don't suggest restoring
from backup - this disk _is_ the backup.)

The disk is a Maxtor 7245, a 245-megabyte SCSI drive, connected to
an Adaptec AHA-1542 host adapter. I figured that if I could drop
this subsystem into a Unix box and get it to boot, I could do a
mount -t msdos and get at the files. (I have a lot more faith in
*n*x plug-and-play than the Windows version, with experience to
back it up.)

I have a box running OpenBSD3.3 (ASUS SP97-V motherboard with the
onboard VGA disabled, 266-MHz Pentium, 32MB RAM, 6GB IDE hard drive).
I stuck the Adaptec host adapter into an ISA slot and fired it up.
Initial boot messages were encouraging:

...
aha0 at isa0 port 0x330/4 irq 11 drq 5: model AHA-1542C, firmware 0.1
aha0: unlocking mailbox interface
aha0: async, parity
scsibus1 at aha0: 8 targets
sd0 at scsibus1 targ 0 lun 0: <MAXTOR, 7245-SCSI, 1560> SCSI1 0/direct fixed
sd0: 234MB, 1944 cyl, 4 head, 61 sec, 512 bytes/sec, 479656 sec total
...

Hey, it found it! Looking good... at least until, half a screen later:

kernel: page fault trap, code=0
Stopped at _aha_timeout+0x24: cmpl $0,0x20(%edx)
ddb>

Damn. Any ideas where to go from here?

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