When I first got the system, it was virgin, with 4x30Gig. I upgraded the SnapOS to 4.0.830 which was the highest at the time. I then decided to upgrade it to 3x300Gig because I didn't have the other $100 for the 4th 300Gig.
I replaced 3 of the 30's with the 3-300's and the OS found them and formatted them which took about 2 hours. Then I went to configure the disks and selected the 3-300Gigs for RAID 5, and didn't select the 30Gig. Then the server re-booted and took about another hour to format the RAID and about 18 hours to rebuild the RAID. Sharing the RAID and sharing the 30Gig put two different drives on the network. The 30Gig is what I am using to just place all my Snap files on. I have about 6 different Snap servers here at the house.
Since I haven't tried a 4th 300Gig, I can't comment on using it as a spare but since I have other places to use the drive, I am. I just bought another 300Gig HD and I'm putting it in my Snap 1100.
One thing though - I tried Poogle_UK's Server Sync that doesn't need a license key and started copying files to the 4000 from my 2200 and about an hour into the exchange, the 4000 crapped and all the files were lost, RAID missing and even the 30Gig got erased. I can't comment for sure if the hacked Server Sync was the problem but I'm not putting it on again.