So when doing a regular check on my media server I got the device is missing.  I have had this happen before and it was a simple sata cable issue so that's what I assumed this was.  I use a Norco 4020 case with a 6tb parity drive and a mix of 3tb and 6tb HDDs.  I powered down, took my "missing disk" out and verified on my desktop PC that the data was still intact.  So since I knew it would have to rebuild from parity, I went ahead, left things powered down, ordered a replacement drive so if anything went wonky during rebuild I would still have that disc and its almost 6TB of data.  The replacement drive arrived today and before I did anything, I swapped power and sata cables around, rebooted and it recognized a new device.  Now since this was still my original drive, and since I wanted to not lose that data just in case, I powered down, swapped in the replacement HDD and rebooted again, but it the new device is no longer recognized.  I am unsure what else to check but I am pretty sure the issue is hardware related.  I know enough to mess things up but I am thinking about moving everything to a new server case but didn't know if I could do that without losing data.  The contents of the missing drive are being perfectly emulated but since I can't get it to recognize a new device, I can't initiate a rebuild from parity.  

Try swapping cables with a different disk, if it still doesn't show it's likely a disk problem.

Disk works in desktop pc.  Im thinking it is a cable issue.  Im ordering some new sata and will report back.  The norco case is pretty hit or miss with reviews and I've been running for several years so it's probably time to start thinkng about a hardware upgrade.

I ended up replacing the power and sata cables, powered back up and the drive was recognized.  I started a rebuild from parity which finished overnight, no errors and all is well in the world again.  Crazy but cables do go bad.