Hello OzzyJim,
I would recommend against SCSI, once upon a time it was the defacto RAID option but nowadays most SATA motherboards have raid, its all built in. Also SATA drives are so d@mn cheap!
The backup methodologies and systems vary on size, skill and budget.
Personally I don't have any redundancy...haha.. I could lose everything at any time. The only thing I have going for me is that I have my OS installed on a different partition
But then again this isnt my lively hood, and I'm broke!
I'll try to describe a few different ways of doing things....
1. workstations have space on the server. the worker bees may keep some files locally but basically keep their files on the server. the IT peeps back up the server in several ways.... a RAID array..... a backup computer in another part of the building or off site and/or physically removable backup options like tapes or drives.
2. at home you might have a few computers and a server, connected by a fast giga lan. on your workstation you could have 1 drive for your OS and apps. and then 2+ drives in a RAID array and then have a scheduled back up occuring every week or so to your server. On your server you could have the same sort of set up. Then also a syncronizing external hard drive (or optical disks or tape drives but I think the external hard drive is the best option), plug it in every week or so and let it do its magic.
3. Something thats basically just as redundant as #2 is having.... in your workstation 1 hard drive for the OS, 2+ drives for a RAID array and then back up with a syncing external drive.
The external back up solutions are important because there's one thing that you can never recover your data from: fire! You should keep your external backups (drives, optical disks, tapes etc) inside a reliable fireproof safe. If you wake up at night you shouldn't be running into your study to fetch your external drive.
With regards to speed/performance....... As you've noted its not really speedy to work on files that are located on a USB drive or network. But SATA RAID arrays offer great performance, and you don't really need the drives mirrored either, just having your OS on its own disk is going to give you a good boost anyways. Though a 4 disk mirrored RAID array would be swee haha.
It's not clear from your post but do you back up your files onto your external drives? Or do they reside only on the external drives and not on the internal drives? If the former I'd recommend rigging something up internally and syncing them to your external drives due to the higher fail rate of external drives.