I turned on my SS4200 yesterday, as usual, and it booted fine, but immediately the disk light activity was almost constant, so I logged into the web console and saw that the Disks health was "degraded" and it was doing a check of the entire array. Which I let it do over the course of the evening, it completed without incident and now says it's healthy again. All my data is intact.
Is this normal? Or is one my disks faulty?
I'm running the latest Intel firmware and 4 identical 1.5TB drives that not used for anything else previously.
The Event Log may give you some clues. Log in to the web UI, and then go to Settings => Event Log. If you're seeing multiple errors on one hard drive, you should consider replacing it.