Greetings, just for the record, I never managed to solve this problem, but the problem exists with the latest OpenBSD and FreeBSD incarnations of this driver as well as under Linux (for the sake of simplicity Gentoo 2005.0 boot-cd). Switched to an Adaptec HW-RAID and the damn thing is working now.
Best regards and sorry for wasting your bandwidth (again),
Moritz.
