When a Hard Drive appears offline not necessarily is defective or bad. Sometimes the issue is that the RAID structure of the Hard Drive got corrupted. You can access the RAID BIOS of the motherboard, from the RAID BIOS you can set only that Hard Drive as non member disk so; that will delete the RAID structure from the Hard Drive. After that the Hard Drive will be clean, and then in Windows from the Intel Matrix Storage Manager software you can rebuild the RAID 5 volume using that Hard Drive again.
Thanks for the response!
I figured it out on my own - or with the help of Google and some luck. I saw that "Reset Disks to Non-RAID" actually should read, "Reset Disk(s) to Non-RAID". The menu option gives no indication that you can reset just ONE disk - which is kind of a bummer.
The second problem is that the RAID apparently won't rebuild outside Windows and Matrix Storage Manager application... My OS is FreeNAS (Linux) so I can't use that. My only option was to borrow an old hard drive from work and install Windows 7 on that, then install the Storage Manager application and wait for the RAID to rebuild. (2 days later it's at 95% of 5.5TB... lol)
So hopefully I can let the RAID get sorted, then shut down, change hard drives back to FreeNAS and go from there.
I guess the lesson learned here is to get a real RAID controller card. I've always used Intel's on-board solution but haven't had a RAID failure until now. It's just not robust enough - at least if you aren't running Windows.