Very many thanks to all who responded - having followed some of the links provided, I now have a much better understanding of RAID technology - .
And at the end of the day, I actually wonder whether I actually need it!
My requirements are for file integrity, not performance - hence the RAID set has been configured for the one drive to mirror the other - RAID1 as I have now learnt! None of the files are big, but there are a lot of them - about 30 000 - so striping isn't going to help.
But it does seem to me that I would actually do better to configure the drives as non-RAID set and use Acronis True Image (as suggested by Richard) to maintain a daily back up of all my files. That way I can roll back out of the sort of situation I have hit today, whereby all of a sudden, my system doesn't want to see any of my card readers for CF and SM cards. Don't quite know what happened, but even when I uninstall and re-install - it appears to uninstall and then re-install OK - but Win Explorer doesn't see any of the drives.Bah!
One other comment I would make - for what it's worth - is that an external drive connected by USB is also somewhat fragile - de-fragging a 40GB drive took about ten hours, and that was in addition to about eight hours to run CHKDSK (which found and corrected about a dozen cross-linked files.
Anyway, once again for all the help ...