I had a powercut last night in the early hours of the morning for like, a few seconds.

Since then I've turned my server back on and now one of my disks is getting the ''Device is disabled, content emulated' message. Is this drive as good as dead now?

It's a possibility, but not likely.  Please see Need help? Read me first!, and attach the diagnostics zip.

I've attatched the syslog here.

The bad drive is the Z1F0R7LK drive.

 

I had a period of about a year without a server. I only started again with it in December I think. They are all drives I've had for a couple of years.

I wouldn't say they have had heavy usage or anything. They're just used to copy films to

Happened with another drive I had (that's no longer in the server), in January I think.

tower-diagnostics-20160327-2349.zip

You will want to read the Troubleshooting section What do I do if I get a red X next to a hard disk?  I think Disk 3 falls into the Drive had faults group, still may be usable but needs repair first.  Disk 3 currently has 8 bad sectors, and has had 104 previously, so does not look particularly reliable.  I assume you already know that that model of Seagate (ST3000DM001) happens to have the worst reliability record of all hard drives currently sold, as far as I know.  They sold very cheaply, so there are a lot of them out there, but appear to have a short life.  Yours may have more life in it, but is not going to get any better!

 

Your 2 choices are -

* Unassign it, Preclear it 2 or 3 passes, and if good, then reassign it and rebuild Disk 3 onto it.  Disk 3 will be emulated during this period, so the array is in a degraded state.  This is not recommended, plus there's a chance the disk won't pass the Preclearing.

* Purchase a replacement disk, Preclear it, then assign it to Disk 3 and rebuild.  This is the safer and recommended choice.

As Rob noted, the SAFE choice is to buy a new drive and rebuild the failed drive onto the new drive.

 

You can THEN thoroughly test the drive that failed, and if it seems to be okay you can consider adding it to the array ... although candidly I consider drives in that category good for backups, but not for active use in the array.