Disk missing, don't understand message

[n8n]

Hello.
It's been some time since I logged into my system. When I did tonight, I was greeted with a raid degraded status, and a message under
Disks-Pools-ZFS-Information

"One or more devices has been removed by the administrator."

The log files don't indicate when the removal occurred. I don't think I removed the disk.
Does this message mean that the disk has failed? Under Disks-Management, the serial number and model are n/a, and editing it has only greyed out options.
I'll keep searching the forums for more info: I'm just concerned that the second disk will fail before figure out what my next step is.
Thanks in advance for any advice.

[Lee Sharp]

A disk is totally disconnected from the system. This could be nothing more than an unpluged cable, or it could be a totally failed disk. Print the disk list, and then look at your drives. You will have serial numbers for everything it sees, and the one not listed is disconnected, or broken.

[n8n]

Thanks for replying.
My system isn't very advanced - 2, 2TB drives.
I shut down the system and tried booting up with each drive disconnected. One drive would boot, with the other, the bios for the raid controller wouldn't let the machine boot up, telling me there was an error in the configuration. Once I had the system booted up again, I confirmed the s/n of the drive that booted with what was showing as online in the pool, and a new hard drive later, the resilvering is complete. N4F under zfs is giving me an error saying that the wrong disk is in the system - I used the replace command in the tools gui for zfs, so I need to read a bit more about that message.
The failed drive is a WD Red, "NAS rated" drive with a 3 year warranty. It lasted a year and 4 months. At least I have the receipt....

[ChriZathens]

Before returning back the drive, also check the -supposed- problematic disk with different power/sata cables to make sure that it is not a cabling fault.