> I have a sun 3/50.  I cannot boot from (external) scsi drives.  here
> is a sample session.

> > b sd(0,0,0)
> scsi select failed
> Device not found
> >

Are you sure you've got the disk on SCSI ID 0?

> I have previously installed a diskless setup, e.g. I can boot from
> le(0,0,0).

> I have disklabeled the disk and created a filesystem => the disk
> works!

This sounds even more as though the disk is not on ID 0.  Look at the
boot messages when you boot diskless; you'll see something like "sd0 at
scsibus0 targ 3 lun 0: ....".  The number after "targ" is the ID the
disk is on, and I expect it's not zero.

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