BenV's notes

It just took me about an hour to figure this one out, so here’s the story for the next time I run into it.
Steps taken:
* New machine
* 2 harddisks (Western Digital Greens, so used wdidle3 on them!)
* Boot Slackware64 installer from PXE/NFS
* cfdisk, create 2 identical partitions, make bootable, set type to FD, write, quit
* mdadm –create /dev/md0 –raid-level=1 –raid-devices=2 /dev/sda1 /dev/sdb2
* install slackware64, grub2 and some other junk
* reboot

Sounds good right?
Well, for some reason the array kept booting up with only 1 of 2 disks active.
No errors or warnings, just kept fucking up. /proc/mdstat looked like

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid1 sdb1[1]
39061952 blocks [1/2] [_U]

Adding /dev/sda1 back (mdadm /dev/md0 –add /dev/sda1) worked fine too, the array resync-ed without problems.

After about an hour of trying to recreate superblocks and that sort of stuff I found it:
The partition type of /dev/sda1 was set to 0x83 instead of 0xFD.
Thanks cfdisk, last time I used that piece of garbage. (I’m 100% certain I set them to 0xFD, but somehow it’s bugging for me lately in cfdisk).