I have a home server, which runs on an old desktop computer from 2009 or so. This office PC, despite being quite old, has very quiet fans, basic onboard VGA, onboard SATA, gigabit Ethernet and 4GB of RAM making it a very respectable home server.

After upgrading the Linux distro on it I noticed a few days later that I couldn’t upload files to it. I checked dmesg and noticed BTRFS errors spewing past about disk writes timing out or something. I promptly shut down the server and ordered two replacement hard drives so I had somewhere to attempt to dump data whilst salvaging what I presumed to be a disk failure or a bad kernel or something.

I ordered an SSD to replace the system HDD at the same time.

When everything arrived I used my PC to image the system HDD to the SSD and booted the server up on its new SSD with the data partition removed from fstab; the data disks were still plugged in. It got stuck on boot and started spewing the same error messages. I tried each data disk on its own and still I got the same errors.

Oh dear. Perhaps both my mirrored drives had given up the ghost simultaneously!

I plugged the data drives into my desktop and booted up: they worked just fine. I booted the server up with just the SSD and it booted. When I tried it with the two new data disks, it failed. It was at this point I assumed that the motherboard was fried and started looking at purchasing a new home server.

There was one final thing that didn’t make sense: during boot the BIOS was complaining how the case intrusion detection switch had fired: I was sure that I had disabled this feature in the BIOS. I checked further and all the BIOS configuration was reset. I noticed one final message in the BIOS logs:

Invalid BIOS configuration, system battery low

I’ve never had to replace a BIOS battery before, but I guess they do run out after about 11 years or so. I swapped the battery with the battery in the kitchen scales and lo and behold, the BIOS didn’t complain and the kitchen scales didn’t work. I turned off the case intrusion setting, set the boot drive order and booted up. The error was still occurring.

It was at this point that I looked at the stack trace a bit more:

INFO: task btrfs:312 blocked for more than 120 seconds.

The floppy disk driver? What?

Now yes, my home server used to have a floppy drive. I had removed it: you can put hard drives in floppy drive bays in most desktop towers. I (re)-disabled the floppy drive in the BIOS and everything booted as per usual.

What I can only assume was happening was that the floppy controller on this motherboard can’t detect if there’s no floppy drive connected. Presumably the line in the floppy cable indicating if a disk is inserted was floating “on” and the motherboard was trying to step the motor to read sector 0.

Why was this only found when I upgraded the server’s OS? After I did that I unplugged it to vacuum the case out. It had been plugged into mains for several months before that.

Why did the server boot with only the SSD? No idea. Perhaps with only 1 working disk and 1 I/O-blocking “floppy drive” the kernel had enough threads to not get all of them stuck waiting.

The good news is I now have a bit more storage space on my server :)