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    "I had to reboot my server and thus stop the array after ~5h an the second time my sdb drive mysteriously disappeared" -- well, that sounds like it could include just about anything, so I'd probably go with just redoing the whole thing from start. FWIW, if you're aiming for a three-drive array in the end, but need one to hold the data temporarily, I'd create the array with three drives from the start (-n 3) and just list one of them as missing. You can add the missing drive later on. Commented Dec 31, 2017 at 18:47
  • @ilkkachu My problem is just that I have no idea how to continue the sync process. -n 3 doesn't work with just two drives Commented Dec 31, 2017 at 22:04
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    strongly recommend you don't do this. btrfs still doesn't have a reliable raid5/6 capability, and btrfs on top of mdadm raid5 is no safer. recommend ZFS instead (most distros have it packaged now, including dkms packages for the kernel module). You can create a zpool with a missing vdev by creating a sparse file of the appropriate size using truncate -s, then zpool create ...., and then zpool offline the sparse file. zfs create a fs on the pool, rsync your sdd data to it, unmount the sdd filesystem then zpool replace the sparse file with sdd. Commented Jan 1, 2018 at 10:32
  • @cas I've never had problems with btrfs, and since ZFS is too demanding for my server (4GB RAM), I think md(adm) is the most secure and reliable solution for me. Commented Jan 2, 2018 at 23:42
  • It's your data, you can trust it to whatever you want, but IMO you should thoroughly research the risks of btrfs raid5/6 first (the btrfs devs warn that it's only for testing), and research the various alternatives too. BTW, just as with zfs, if you don't give individual drives/block-devs to the fs but instead layer btrfs on top of either hardware or software RAID, you lose the most important benefit of using the fs, error correction. Commented Jan 3, 2018 at 0:38