Hi Graeme, welcome here
I guess you simply install SMe on a single hd without any boot parameters, so you should aleready have a "degraded" single disk Raid1
I strongly suggest you to think about a dual 500 Gb solution: the hd will be setup in a raid1 array, so you'll get about 500 Gb available
so:
1) plug in the first empty hd (of course, you have to shut down your server

)
2) go to admin console and choose "manage disk redundancy"
3) it should tell you you have and empty disk and ask if you want to add it: do it
4) let the array sync. it could take some time according to your hd size
5) shutdown your server, unplug your original hd, plug in the second empty hd
6) reboot, go to console again and repeat step 3
7) let the array sync

follow the instructions you find
here, in the box
it should work (at least it worked for me many times)
please take some time to read carefully the documentation and to search the forums before posting, thank you
finally, if you find something that doesn't work as expected, please raise a bug in bugzilla (link is above)
