Good luck! After going down this road this past week (unsuccessfully I must add) I will read your progress with great interest. Also, I'm still looking for my bios and Snap OS upgrade files, if I find them I'll let you know. A quick couple of things that may save you some time:
1. Locate the homepage for your replacement HD's before you purchase them. Ensure all their particulars are correct, and that it is easy to get their utility tools to downgrade the drive ATA speed if necessary.
2. Always unplug the Snap power cable before doing any internal work. I acted too quickly a few days ago, and blew the fuse when something accidentally touched something else. i was lucky that it was only the fuse that blew. The power switch is only a "soft switch" and doesn't turn off the juice.
3. Print out all your Snap admin settings using the "hidden config" page.
http://snapservername/config/debug/ -- you know what to do with snapservername ...
4. Backup all your Snap data to another harddrive first - probably not a biggie since you just got the server.
Good luck!!
Nigel