Can't overwrite a particular swap partition

[RW]

I have two swap partitions, ad4s1b is part of my main slice, and ad6s1b
is on a second drive. The permissions are the same, but I can't
overwrite the second one. The same thing happens whether I use swapoff
or reboot into single user mode.

What's the difference?

# dd if=/dev/random of=/dev/ad4s1b bs=1m
dd: /dev/ad4s1b: end of device
4097+0 records in
4096+0 records out
4294967296 bytes transferred in 245.745739 secs (17477281 bytes/sec)

# dd if=/dev/random of=/dev/ad6s1b bs=1m
dd: /dev/ad6s1b: Operation not permitted
1+0 records in
0+0 records out
0 bytes transferred in 0.053829 secs (0 bytes/sec)

# ls -l /dev/ad*1b
crw-r-----  1 root  operator    0, 125 Oct 25 00:25 /dev/ad4s1b
crw-r-----  1 root  operator    0, 132 Oct 24 20:23 /dev/ad6s1b