mdadm: super0.90 cannot open /dev/sdb1: Device or resource busy mdadm: /dev/sdb1 is not suitable for this array.

mdadm --create /dev/md0 --level 1 --raid-devices 2 /dev/sdb1 missing --metadata=0.90
mdadm: super0.90 cannot open /dev/sdb1: Device or resource busy
mdadm: /dev/sdb1 is not suitable for this array.
mdadm: create aborted

Sometimes running "partprobe" can fix this.  Other times it requires a reboot.

One other manual thing that can be done is the following to fix it (if dm is using and blocking it):

dmsetup table

Then remove the entry that is using /dev/sdb1

dmsetup remove [the device id from above]

If the above doesn't work could it be that you used -f to fail a partition to test or by accident?

md3 : active raid10 sdb3[2](F) sda3[0]
      436343808 blocks 512K chunks 2 far-copies [2/1] [U_]
      bitmap: 4/4 pages [16KB], 65536KB chunk

mdadm: Cannot open /dev/sdb3: Device or resource busy

Fix it by removing the device from the mdadm array

mdadm --manage /dev/md3 -r /dev/sdb3
mdadm: hot removed /dev/sdb3 from /dev/md3

Now we can readd it without any error:

mdadm --manage /dev/md3 -a /dev/sdb3
mdadm: re-added /dev/sdb3


mdadm, dev, sdb, resource, suitable, array, md, raid, devices, metadata, aborted, quot, partprobe, requires, reboot, manual, dm, blocking, dmsetup, entry,

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