root@irina [~] # dd if=/dev/zero of=/dev/rsd0a bs=32k
dd: /dev/rsd0a: Read-only file system
1+0 records in
0+0 records out
0 bytes transferred in 0.001 secs (0 bytes/sec)
root@irina [~] #

Well, it is not even mounted. But since it contains a valid
system, it will mount (rw!) without problems.

Furthermore, an "fdisk sd0", "disklabel sd0" will either give a
"Read-only file system" on the "d" slice (this is i386) if
the "a" slice  is not mounted yet, or a

disklabel: Invalid signature in mbr record 0
disklabel: ioctl DIOCGDINFO: Inappropriate ioctl for device

if the partition is mounted.

Hm ... why doesn't that work / why can I still mount that disk???

I'd like to wipe off that disk entirely.

I'm really confused :-(

Thanks
Florian