That drive I foolishly picked up used on Amazon and which caused some issues right off the bat as chronicled in this thread, and which I got to "behave" for a while after a lot of help from the community, threw 4 errors again a few weeks ago and was disabled by unRAID.

Being that the server is in Berlin and I in Los Angeles for the next 6 months or more, I stopped the array, un-assigned that disabled disk 3, started the array again, stopped it, re-assigned it, and waited until its content was rebuilt. Everything seemed fine for a few weeks now, but during a sync operation today from an iMac at my apartment in Berlin to my server, which is hosted at a friend's web-agency in Berlin, the sync process got hung up and when I looked into the cause of it, it turned out that disk 3 showed 4 errors and was disabled again.

My gut feeling is that the drive is bad, but even if it is, is there a way to "fix" it by taking the offending sectors/blocks out of commission, so to say, or is that what unRAID or the drive's firmware or whatever is doing in the background already, and once these errors are being reported by unRAID the way they are now and the drive disabled, is that the point at which point it needs to be taken out of commission entirely?

I included the unRAID diags and the SMART report, for all it's worth. 
  

unraid-diagnostics-20181008-0444.zip

WDC_WD60EFRX-68MYMN1_WD-WXL1H642H7PL-20181008-0442.txt

6 hours ago, tillkrueger said:

My gut feeling is that the drive is bad,

It is, a rebuild can re-write the bad sectors and it can then work for a little while longer, until it fails again.

Thanks for confirming, jb.