Back to the old drive.
Looks like Microsoft Entourage is broken in Mavericks, and very worryingly, doesn't want to allow me to archive all my emails, only saving stuff up to about 3 years ago - will have to try again. The issues of poor performance seemed to still be present, even in a fresh install, even when Spotlight had finished archiving. I'm about to create a Mountain Lion install flash drive in the hope that will help fix things. Failing that I'm not sure where we go from here, other than I'm going to be flogging some recently used kit.
The idea of leaving the world of Apple is looking attractive again.