The MOST LIKELY (based on my prior experience with Dell systems) problem is that something hiccuped on the SCSI bus knocking two drives off line. Go into the RAID controller and try to force them both back online. If you're not sure how to do this, call Dell support (It's been a while since I've had to since I stopped using RAID 5 in part because of this). Ultimately, you could have a failing drive, failing controller, or even failing cable (I've seen cables make a difference). But in my experience, 50/50 odds that this is a random issue that won't recur again for months (which stinks since you'll probably have it happen again, but so infrequently you can't easily diagnose what's causing it).
