So Far So Good
The RAID is a linux software raid. I can't find a menu on the Dell to implement a hardware raid, which would have been my preference.
The OS is now running fine, and I am rebuilding the SAMBA server so that the various windoze clients can see their data again.
One issue is that because I mounted one of the drives as /tmp, there are lots of processes now using it, so the OS won't let me unmount it! Lesson to be learnt there, methinks. Any ideas on how to get out of this one?