I am working on 2 mono slabs that I would like to finish restoring. While I do not have a NeXT external CD-Rom, I do have a few other external CD-ROMS in my Mac collection. I do have the origional NeXTStep 3.0 media and CD-Rom boot floppy.
I will try using a couple of them this weekend, but thought I would post the question quick before I continue. I also have a color slab and a cube that are running (got them with OS installed), so it isnt a huge rush to get these slabs running.
Take a look through here for starters. https://www.levenez.com/NeXTSTEP/faq.html
My goto SCSI2 drive is a seagate ST15230N. Many of us also use the SCSI2SD adapter (v5) for SCSI instead of physical spinning rust.
If you have NeXT machines running already, you can create "factory fresh" hard drives using BuildDisk. Hook one up externally and then use BuildDisk, then pop that external into the new machine.
Unless, of course, you really want to go through the whole CD based install :-)