As an update to this request for opinions, I’ve now received the USB enclosure for the for the spare 60GB SATA HD and put the two together. I’ve populated this device with what was on my spare 64GB PATA SSD, namely installers for different flavours of MacOSX (from 10·4 to 10·8), then copied the contents of Pismo’s 32GB PATA SSD to the 64GB PATA SSD. (This took all night because Pismo only has USB 1·1 – slow!)
Just now I’ve put the 64GB SSD into Pismo – she seems to be working fine. In fact she seems a little snappier than before. And so I now have a ‘spare’ 32GB PATA SSD in a USB enclosure. (I’ll leave it alone for now – it has a known-good copy of Pismo’s OS and apps, just in case the 64GB SSD gets flaky.)
Pismo now has 33GB free (3GB on the OS9 partition, 30GB on the OSX partition). I seem to use about 2·5GB per year, so this space may well do for another 10 years.
The final steps would be to put WinXP back on the netbook and find a suitable home for it. Oh, and stop myself wasting money reviving the macbook pro. (It’s been prevented this month by £100 drop in my income and buying running shoes.)