This is on a 7500, with the CD-ROM plugged into the "external" NCR SCSI
bus. I can load almost any kernel from floppy or a SCSI drive on either bus
(MESH "internal" or NCR "external").  Once it gets to the point of probing
the SCSI busses for devices, it always fails in pretty much the same way:

scsibus0:waiting 2 seconds for devices to settle...
probe(esp0:3:0): esp0: timed out [ecb 0xe02210d0 (flags 0x3, dleft 0, stat
0)], <state 4, nexus 0xe02210d0, phase(I 16, c 6, p 6), resid 0, msg(q 0,o
0) DMA active>
probe(esp0:3:0): esp0: timed out [ecb 0xe02210d0 (flags 0x43, dleft 0, stat
0)], <state 4, nexus 0xe02210d0, phase(I 16, c 6, p 6), resid 0, msg(q 20,o
0) DMA active> AGAIN
esp0: SCSI bus reset
probe(esp0:3:0): max sync rate 5.00MB/s
probe(esp0:3:0): esp0: timed out [ecb 0xe02210d0 (flags 0x7, dleft 20, stat
2)], <state 4, nexus 0xe02210d0, phase(I 41, c 1, p 1), resid 20, msg(q 0,o
0) DMA active>
probe(esp0:3:0): sync negotiation disabled
probe(esp0:3:0): esp0: timed out [ecb 0xe02210d0 (flags 0x47, dleft 20,
stat 2)], <state 4, nexus 0xe02210d0, phase(I 41, c 1, p 1), resid 20,
msg(q 20,o 0) DMA active> AGAIN
esp0: SCSI bus reset
probe(esp0:3:0): Sense Error Code 0x5f at block no. 2358781 (decimal)
sd0at scsibus target 3 lun0: <, , > SCSI0 0/direct fixed
sd0: mode sense (4) returned nonsense; using fictitious geometry
(esp0:3:0): selection failed; 4 left in FIFO [intr 18, stat 93, step 3]

[...]

Ultimately, it lets me plug in a root device, but the CD-ROM is unusable,
as it has a fictitious geometry.  Even worse is are the latest GENERIC_MD
kernels (from the boot.fs floppy and the netbsd.hfsboot kernel) which don't
even recognize that there is a sd0 -- they just give up.  And, BTW,
shouldn't the CD-ROM show up as cd0?

The kernels I tried are:
<ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-1.5_ALPHA2/macppc/binary/sets/kern.tgz>
<ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-1.5_ALPHA2/macppc/installation/floppy/bo
ot.fs>
<ftp://ftp.netbsd.org/pub/NetBSD/arch/macppc/snapshot/20000620-1.5/binary/kernel
/netbsd.GENERIC.gz>
<ftp://ftp.netbsd.org/pub/NetBSD/arch/macppc/snapshot/20000620-1.5/installation/
netbsd.ram.gz>
<ftp://nandra.iri.co.jp/pub/NetBSD/macppc/netbsd.hfsboot>

The CD-ROMs I tried are a Teac 12x from a PowerComputing PowerCenter Pro
and an "Apple" brand 12x from a 7300.  I've never had any other problems
with either of these two CD-ROM drives.

Any suggestions?

  -- MW