mdadm --manage /dev/md125 -a /dev/sdb4
mdadm: add new device failed for /dev/sdb4 as 3: Invalid argument

Dec 19 01:23:48 storageboxtest kernel: [ 2328.854491] md: sdb4 does not have a valid v1.2 superblock, not importing!
Dec 19 01:23:48 storageboxtest kernel: [ 2328.854499] md: md_import_device returned -22

#zero the superblock and try again!
mdadm --zero-superblock /dev/sdb4
mdadm --manage /dev/md125 -a /dev/sdb4

mdadm: add new device failed for /dev/sdb4 as 3: Invalid argument



Solution / this is the only thing that fixed it (basically do more than just zero-superblock)

dd if=/dev/zero of=/dev/sdb4
^C77149+0 records in
77149+0 records out
39500288 bytes (40 MB) copied, 5.35553 s, 7.4 MB/s

#success!

mdadm --manage /dev/md125 -a /dev/sdb4
mdadm: added /dev/sdb4
 

The weird thing is that you can see zero-superblock was NOT enough to fix it and sometimes you have to wipe a little further than zero-superblock does and it will fix it.  I didn't find this solution anywhere so I thought I'd share it.

mdadm, dev, sdb, invalid, solutionmdadm, md, dec, storageboxtest, kernel, valid, superblock, importing, md_import_device, dd, bytes, mb, copied, didn,

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