'ello!

I just wanted to 'scrubb' my new external USB-Harddisk..

********************************************
umass0 at uhub0 port 1 configuration 1 interface 0
umass0: Western Digital External HDD, rev 2.00/6.02, addr 2
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, 1 lun per target
sd0 at scsibus0 target 0 lun 0: <WD, 1600BB External, 0602> disk fixed
sd0: fabricating a geometry
sd0: 149 GB, 152627 cyl, 64 head, 32 sec, 512 bytes/sect x 312581808 sectors
********************************************


..with..

********************************************
cgdconfig -s cgd1 /dev/sd0a blowfish-cbc 128 </dev/urandom
dd if=/dev/zero of=/dev/rcgd1d bs=64k
********************************************


..but unfortunately, I get this:

********************************************
petar@starling
$ sudo dd if=/dev/zero of=/dev/rcgd1d bs=64k
dd: /dev/rcgd1d: Read-only file system
1+0 records in
0+0 records out
0 bytes transferred in 0.003 secs (0 bytes/sec)
********************************************
petar@starling
$ dmesg | tail
(...)
cgd1: error 30
********************************************


A quick Google on mail-index didn't help and I really don't have a clue, 
what I'm doing wrong..


Thank you very much & with kind regards,

Petar