Thank you Mike for the explanations.

Two arrays with a mirror from one to the other does sound pretty good, something that I will probably work on once one of my existing pc's gets retired to another naslite machine.
The external hdd's that I'll be using shortly 1/pc will be backed up on the naslite, it was just a matter of at the moment I have 2 hdd's in the nas, two external and two pc's, so each pc has all its data mirrored twice, to external drive and to nas drive.
Now I'm looking at three pc's to mirror, twice, I need another ide hdd in the nas and keep it simple, or switch over to sata drives with a sata controller.... just wondering if something like the the AMCC 3Ware 9500S-4LP SATA Raid Controller 128MB 4 Ports would allow me to have 3 drives all with their own labels so I know which is back up drive is for what machine?
Cheers
DC