My God, the command is not working.  I need to remove sdb1 from md0 so I can change it from a RAID10 to RAID1, and it simply ignores my command:
# mdadm /dev/md0 --fail /dev/sdb1 --remove /dev/sdb1
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md2 : active raid10 sdb3[1]
      1868560128 blocks super 1.2 64K chunks 2 offset-copies [2/1] [_U]
      bitmap: 94/446 pages [376KB], 2048KB chunk

md1 : active raid10 sdb2[1]
      6297344 blocks super 1.2 64K chunks 2 offset-copies [2/1] [_U]
      bitmap: 0/25 pages [0KB], 128KB chunk

md0 : active raid10 sdb1[1]
      78654080 blocks super 1.2 64K chunks 2 offset-copies [2/1] [_U]
      bitmap: 76/151 pages [304KB], 256KB chunk

unused devices: <none>
#


My system is half-converted and is now unbootable.  What am I going to do?