I stopped the array because I wanted to add a disk to the array. But after stopping the array one disk position showed as unavailable. I probably did something stupid and started the array probably hoping the situation would be reversed. Off course it wasn't and the position showed now as not assigned. I then powered down the server, reseated cables, powered back up and found the same situation. The disk position showed as not assigned as expected, but now the missing disk showed up when pressing the little arrow next to the box. Can I re-assign the disk and let it rebuild, should I replace the disk or can I re-assign the disk and use the trust array method? Come to think of it, what exactly is this trust array method and how do I do this?

I think it's actually labelled Trust parity. It means that parity won't be rebuilt. You should not use it unless you are sure parity is still valid.

 

Does unRAID want to rebuild the disk? If so then you should let it. If any writes happened while the disk was missing then those writes would not be on the disk, but they would be on parity, so a rebuild would get them on the disk.

Does unRAID want to rebuild the disk? If so then you should let it. If any writes happened while the disk was missing then those writes would not be on the disk, but they would be on parity, so a rebuild would get them on the disk.

I am surprised that unRAID did not complain about a "missing disk" when it was started without one assigned.  It will be worth checking that unRAID is offering to rebuild the disk before starting the array, as otherwise it might think it should wipe the current contents which is not what is wanted.

Yes, I saw the message next to the start button that when I start the array, data rebuild will start and the file system will be expanded if possible. So I did and rebuild is in progress. Now I am wondering what caused  this. I don't think I should worry about the PSU, it is a Seasonic X760 that is specified for 63 Amps on the 12 V line. It would break down when rebuilding too, with the 21 disks I have now in the server all spun up.

... I think it's actually labelled Trust parity.

 

While it's often referred to as the "Trust parity" option, it actually is a checkbox that says "Parity is already valid" 

 

If you check that, the system won't do a parity sync => but it WILL, when started, begin a parity check.  If you're CERTAIN you were correct, you can safely stop that check.

 

 

... I am wondering what caused  this.

 

Hard to say, but usually it's because a cable has become insecure or the connection in a drive cage (if you're using hot-swap cages) has corroded a bit (or been moved by a slight bump of the case).

 

Reseating the cables and/or unplugging/replugging the drive will often resolve it -- as it did in your case.

 

You COULD have done the New Config w/Trust parity option and all would have been fine => but simply rebuilding the drive will work fine as well ... and since that's in progress, just let it complete.

 

I agree your power supply has plenty of power for your system.