Rebuilding Raid-arrays

That drive sdb looks like it's not far from failing. Though it hasn't officially failed yet, it doesn't have much life left in it.

195 Hardware_ECC_Recovered  0x001a   032   026   000    Old_age   Always       -       38042073

This drive has had a large number of recoverable read errors. Which means it successfully reconstructed the data using error correction. However, it's getting to the point where it is most likely it will soon have an unrecoverable read error, where it cannot successfully reconstruct data on a damaged or failing section of the disk. At that point there's nothing you can do and you'll have to replace the drive.

If your rebuild keeps stopping, at the same place, it's entirely possible the drive has already failed at that point on the platters, and isn't reporting it. Desktop class drives will stop and try for minutes or even hours to read a particular sector if they fail the first time, which leads to this sort of thing. And you probably have such a drive in this "server"...

At this point you should have that drive proactively replaced, since it is going to fail soon, if not already.

It' seems that only one half of the mirror is missing. So there should be no problem.But the question is why are the sumbirrors (sdbX) missing? Maybe it would be a good idea to check them before reatachment to the mirrors.

mdadm --manage /dev/md1 --add /dev/sdb2
mdadm --manage /dev/md2 --add /dev/sdb3
mdadm --manage /dev/md3 --add /dev/sdb4