- really, REALLY noisy
Ugh, I know what you're talking about. Back in the day disks were much louder, even when new. When worn out they become absolutely intolerable.
I have good experience with late model Seagate 15K disks (15K.3 and newer). They have FD bearings so they're basically as quiet as a modern SATA disk. Seek noises are a little louder, but not annoying. YMMV, others like Maxtor. I can find the Seagates easily, so I've standardized on them. I started with the newer systems, but when I recently moved my Power Challenge I found it had two 36GB 15K.4 disks installed
The external SCSI box is easy (I have one with a pair of 300GB Seagate 10K.7 disks). If you want to have a dedicated system disk in each system you can use the external disk box for user data ($HOME). You'll still need a handful of SCSI cables (50pin, 68pin, VHDCI maybe) to attach it if you have multiple generations of systems around. Additional benefit: often, faster SCSI adapters were available as an option, e.g. an Octane has UW SCSI (40MB/s) internally, but it can use a dual channel U160 adapter in the PCI shoebox/shoehorn.
If you really want large storage volumes (>= TB), use the network. 100Mbit adapters exist for Indigo2's, gigabit for Octane & newer etc. A simple Linux box with a couple of TB SATA disk makes a pretty capable NFS server, a NAS will do the same thing (slower, but often cheaper and less power hungry). With more than two systems around you will quickly get tired of the 'on which system is that file' shuffle and get some form of network storage anyway.
- Even wackier: Could you boot THE SAME Irix installation on different hardware platforms? (i don't really want this, but would be a cool hack)
No. It's possible to boot an IRIX installation in another system, as long as it has the same IPnn number. The system disk of an O300 will work in an O350 or Tezro; they're all IP35. Ditto for an Indy and a Challenge S (both IP24) etc. It will probably rebuild the kernel to match the installed hardware, but otherwise it's painless.