On 05/03/20 14:53, Jinpu Wang wrote:

"mdadm /dev/md0 -a /dev/sdl1"  should work for you to add the disk
back to array, maybe you can check first with "mdadm -E /dev/sdl1" to
make sure.

Or better, --re-add or whatever the option is. If it can find the
relevant data in the superblock, like bitmap or journal or whatever, it
will just bring the disk up-to-date. If it can't, it functions just like
add, so you've lost nothing but might gain a lot.

Cheers,
Wol