Well, I don’t think my problem has anything to do with the DawiControl card anymore. I did a little experiment today. I created a 1TiB EXT3 file system on a single drive (one of the new 1TiB drives obviously) and started syncing data over to it (roughly 800MiB).
Now, then I unmounted the drive(s), ran fsck -C -f /dev/sd${deviceletter}1 and it went through without any trouble. Then I removed the partition and created a 1GiB partition on each drive, which I then used to build a new device mapper RAID5 array (with EXT3 on top …).
And guess what happened after I copied the data over, unmounted the file system and ran fsck ? Sure, same thing as yesterday. Now, this means either it’s a mdadm bug, while creating the array or really MD’s fault (which I can rule out, since the same happens on 2.6.25 as well as on 2.6.28) … *shrug*