At 19:00 I receive a notification: the backup server has problems. I log in and check: a drive had died. No big deal; since it's a RAIDZ1, the system kept running. I had already copied the EFI partitions and set things up, so it would be able to boot from the other drives as well. I request a replacement from Hetzner, which they carry out in less than half an hour. Despite being hot-swappable, the server detects the disconnection of another drive and crashes. At that point, I ask them to look into it, and they test the machine. Reboot: it won't start. I request a KVM console. I get it: I had forgotten to update the fstab and it was trying to mount /boot/efi from ada0p1, but ada0 was the replaced drive, and it wouldn't go any further.
Fixed the fstab, recreated the partitions, rebooted, and issued the ZFS command for resilvering.
Result: resilvering in progress and backups working again.
I can turn off the computer and start my Friday evening.
