Thanks a LOT!
That did it.
Actually I got a
hot remove failed for /dev/sdc No such device or address
first, which brought me to http://www.mail-archive.com/linux-ra.../msg01930.html
As it seems re-booting/waiting during the process made mdadm remove the drive completely from the array during the process. So, when I tried to "--remove" it, it was in fact already gone. So
sudo mdadm /dev/md0 -a /dev/sdc
did the trick!
And when resync was slow (7000min! on a non-production machine) I did "sudo -s -H" to circumvent the ubuntu “permission denied” problem and then "echo 50000 >/proc/sys/dev/raid/speed_limit_min" - and instantly I was down to 240min.
Now my last question: Should I worry about fdisk giving
sudo fdisk -l /dev/sdb
Disk /dev/sdb doesn't contain a valid partition table