[Smith]

'Error reading bootsector', this somehow to me suggests that it's not using my painstakingly added openbsd disklabel..

Now I am quite sure there is something fundamental I am missing. I googled around a bit and used ktrace/kdump on the utility ntfsinfo (which produces a similar error). It turned out the point of failure is a call to pread() to get the boot sector, which is where the 'invalid argument' error is returned from. Now I realize I can't even access the first sector as root using dd either:

hostname [~] # dd if=/dev/sd0c of=test.bin bs=512 count=1
dd: /dev/sd0c: Invalid argument
0+0 records in
0+0 records out
0 bytes transferred in 0.000 secs (0 bytes/sec)
hostname [~] # dd if=/dev/rsd0c of=test.bin bs=512 count=1
dd: /dev/rsd0c: Invalid argument
0+0 records in
0+0 records out
0 bytes transferred in 0.000 secs (0 bytes/sec)

Yet mount_msdos seems to manage to read it, and mount the msdos partition which is on the same disk ..

hostname [~] # mount /dev/sd0j /mnt/msdos
hostname [~] # ls /mnt/msdos
System Volume Information
hostname [~] # umount /dev/sd0j