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Debian server not booting after software RAID-1 array degraded

So I have a Debian 7 server with 3 hard drives. Its RAID-1
is basically configured this way:



md0: sda1,
sdb1 --> / (root)
md1: sda5, sdc1 + sdb5 (spare) --> /data (sdc1 is on a
SSD, and sda5 is marked 'writemostly').



both sda
and sdb have grub installed on them.



When
installing an extra network card, I messed up and unplugged sdc's data cable (note that
sdc doesnt have GRUB or /, and should have nothing to do with
booting).




So the system booted fine
after that. I noticed my error, shut down the machine, and plugged sdc back in (while
mdadm was rebuilding md1 on the spare).



Now, the
system either gave me the dreaded GRUB shell, or just a black screen with a blinking
cursor. depending on which hard drive(s) I unplugged. But no combination of hard drives
gave me a successful boot. I also tried it with having all 3 drives connected, and
telling the BIOS to boot from any of the boot drives
manually.



What I did in the end was to boot
Debian setup in rescue mode, aassembled the RAID devices, and let them
rebuild.



This didn't result in a successful
boot.



So I booted rescue mode again, and
manually re-installed GRUB on sda and sdb. This fixed my
problem.




My question is: what
happened here?
a) sdc shouldn't affect booting in any way, AFAIK?
b)
even if something of the RAID rebuilding process I interrupted would affect booting, why
did the system not boot after I rebuild the RAID arrays in rescue mode? Why did I have
to re-install GRUB on sda and sdb manually, if, as far as I understand, the sectors on
the drives that house GRUB dont have anything to do with the RAID
arrays?

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