Now it works!

[Oliver Hoffmann]

> Is this reproducible?

Yes it is.

> Are you sure you have procfs mounted?

Right /proc wasn't mounted. They kicked it out because of it's poor security. 
Didn't knew that.

> 'ls -l' shows size of whole file system.
> du shows the size of the used block in the filesystem
> (files + directories + inode + etc.)

All right.

# truss /sbin/ufs_copy /mnt/fw/testfile /dev/ad0s1g
...
open("/dev/ad0s1g",0x201,0600)                   ERR#16 'Device busy'
ftruncate(0xffffffff,0x0,0x9275e000,0x8)         ERR#9 'Bad file descriptor'
close(4)                                         = 0 (0x0)
exit(0x1)
process exit, rval = 256

I think the problem was that the device was mounted. I made a restore with a 
device wich wasn't mounted and everything is alright now.
Now I can test ufs_copy with my backups and see how feasible it is.

Thanks,

Oliver.