Boot Debian while RAID array is degraded
Solution 1:
You want start_dirty_degraded. Try specifying md-mod.start_dirty_degraded=1 as a boot argument to the kernel image.
Solution 2:
I had the problem that my system would boot normally with /dev/sdb unplugged, but would stall forever if i removed /dev/sda.
Simple solution, after a standard install of Debian, was to simply run grub-install /dev/sdb.
..and now it boots even with /dev/sda disconnected.