#  mdadm --detail /dev/md0 
/dev/md0:
           Version : 1.2
     Creation Time : Wed Sep  9 20:26:10 2020
        Raid Level : raid0
        Array Size : 1953260544 (1862.77 GiB 2000.14 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Wed Sep  9 20:26:10 2020
             State : clean 
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

        Chunk Size : 512K

Consistency Policy : none

              Name : cephdev001:0  (local to host cephdev001)
              UUID : 38561b34:739e61e4:ef8f037b:93e581e9
            Events : 0

    Number   Major   Minor   RaidDevice State
       0     259        1        0      active sync   /dev/nvme1n1
       1     259        2        1      active sync   /dev/nvme2n1

普通にremoveしようとしてもremove もfailもできない。。。

# mdadm --remove /dev/md0 /dev/nvme1n1 
mdadm: hot remove failed for /dev/nvme1n1: Device or resource busy
# mdadm --fail /dev/md0 /dev/nvme1n1 
mdadm: set device faulty failed for /dev/nvme1n1:  Device or resource busy

mdを停止してから、superblockを消してあげればOK

# mdadm --misc --stop /dev/md0 
mdadm: stopped /dev/md0

# mdadm --misc --zero-superblock /dev/nvme1n1 
# mdadm --misc --zero-superblock /dev/nvme2n1