If any device in the array is failed, grub-probe will decide that the md device does not exist. It will then claim that anything on top of this (LVM) also does not exist.
Removed devices are ok, but recovering devices are not. This means that you can't install/update a grub installation while / is on an md device that has failed or recovering devices: # grub-probe / btrfs # mdadm --manage --fail /dev/md0 /dev/sda1 mdadm: set /dev/sda1 faulty in /dev/md0 # grub-probe / grub-probe: error: disk `lvmid/******-****-****-****-****-****-******/******-****-****-****-****-****-******' not found. # mdadm --manage --remove /dev/md0 /dev/sda1 mdadm: hot removed /dev/sda1 from /dev/md0 # grub-probe / btrfs # mdadm --manage --re-add /dev/md0 /dev/sda1 mdadm: re-added /dev/sda1 # cat /proc/mdstat Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md0 : active raid1 sda1[0](W) sdb1[1](W) sdd1[4] sdc1[3] 976629760 blocks super 1.2 [4/3] [_UUU] [>....................] recovery = 0.2% (2473984/976629760) finish=141.5min speed=114688K/sec bitmap: 6/8 pages [24KB], 65536KB chunk unused devices: <none> # grub-probe / grub-probe: error: disk `lvmid/******-****-****-****-****-****-******/******-****-****-****-****-****-******' not found. # cat /proc/mdstat Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md0 : active raid1 sda1[0](W) sdb1[1](W) sdd1[4] sdc1[3] 976629760 blocks super 1.2 [4/4] [UUUU] bitmap: 4/8 pages [16KB], 65536KB chunk unused devices: <none> # grub-probe / btrfs -- Simon Arlott