I have a RAID5 array, like this:

[root@g1016637 ~]# mdadm -D /dev/md127

/dev/md127:
Version : 1.2
Creation Time : Wed Mar 8 16:35:25 2023
Raid Level : raid5
Array Size : 23441682432 (21.83 TiB 24.00 TB)
Used Dev Size : 7813894144 (7.28 TiB 8.00 TB)
Raid Devices : 4
Total Devices : 3
Persistence : Superblock is persistent

 Intent Bitmap : Internal

   Update Time : Thu Jun 13 11:52:49 2024
         State : active, degraded
Active Devices : 3

Working Devices : 3
Failed Devices : 0
Spare Devices : 0

        Layout : left-symmetric
    Chunk Size : 512K

Consistency Policy : bitmap

          Name : cinesend:dcp-data
          UUID : 0fca8296:53b5287c:f3cd71c9:38a2c1b5
        Events : 1100689

Number   Major   Minor   RaidDevice State
   0       8        0        0      active sync   /dev/sda
   1       8       16        1      active sync   /dev/sdb
   -       0        0        2      removed
   4       8       48        3      active sync   /dev/sdd

We’ve replaced /dec/sdc with a new drive, but I can’t seem to add it to the array easily:

[root@g1016637 ~]# mdadm /dev/md127 -a /dev/sdc
mdadm: Cannot open /dev/sdc: Device or resource busy

Previously, these would just get added. I’ve seen this when removing a drive and typically I stop the RAID array, but it appears this error implies the drive I’m trying to add is busy? I don’t see why that would be true since it was just added.