I recently bought a Western Digital USB drive the size of a matchbox that has 500Gb capacity for £75.

How times have moved on!

They’ve moved on but your 500Gb drive isn’t an alternative to the pile of old 36GB SCSI drives because it’ll be very slow in comparison. The SCSI drives are 15K rpm ones and set up as mirror pairs with a database striped across them. That performs ok, despite the drives being old and the processors on the old E250 being about equivalent to a Intel Pentium III. Running the same system on a modern laptop with 4GB of RAM, a Linux VM and an external drive is very slow in comparison.