This is on a 7500, with the CD-ROM plugged into the "internal" MESH 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 one of two ways.  The first
failure method is an endless stream of mesh timeouts.  The other failure
method is a few errors and incorret info about the CD-ROM, but it suceeds
in booting to the point of prompting for a root device.  Here's what the
output looks like:

[...]
mesh0 at obio offset 0x18000 irq13: 50 MHz, SCSI ID 7
scsibus1 at mesh0: 8 targets, 8 luns per target
[...]
scsibus1: waiting 2 seconds for devices to settle...
probe(mesh0:3:0): Sense Error Code 0x0
sd0 at scsibus1 target 3 lun0: <,  , > SCSI0 0/direct fixed
mesh: timeout state=3
mesh: resetting dma
[repeat]
sd0: could not mode sense (4/5); using fictitious geometry
mesh: timeout state=3
mesh: resetting dma

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