1. Get serial of defective disk
# for i in a b; \ do echo $i ; smartctl -i /dev/sd$i | grep -i serial ;\ done
The disk not returning a serial is most likely the defective one.
2. Create a Ticket @ Hetzner Robot
Log in to the Hetzner Robot and create a Ticket: Left menu:
Anfragen -> Serverprobleme ausklappen -> Festplatte defekt
If the serial of the defective drive can’t be obtained, enter the serial of the working one and stress that the said serial is the working drive. Otherwise they’ll swap the wrong one and you end up with nothing!
3. Copy partition table
Once the drive is replaced, copy the partition table from the working drive to the new one. sgdisk comes to rescue (THINK BEFORE YOU COPY & PASTE, WON’T WORK ANYWAY):
# sgdisk -R=/dev/sd[new] /dev/sd[old]
In doubt, RTFM twice: man sgdisk
4. Resync the RAID
mdadm comes to rescue:
# mdadm --manage /dev/md0 --add /dev/sd[new][part]
If you can’t get the former command to work, again: RTFM! It’s your data!
5. Stare at the progress
# watch cat /proc/mdstat