How do you md5 an entire disk in OS X? I want to validate a "dd" image of a USB disk in OS X using the terminal. The terminal command md5 works on the dd image but not on the entire disk.

$ dd if=/dev/disk2 of=image.dd conv=notrunc
1974152+0 records in
1974152+0 records out
1010765824 bytes transferred in 149.033958 secs (6782118 bytes/sec)
$ md5 image.dd 
MD5 (image.dd) = 3efc7e341b1bf2d729ffcd9b6a313428
$ md5 /dev/disk2
MD5 (/dev/disk2) = d41d8cd98f00b204e9800998ecf8427e
$ md5 /dev/rdisk2
MD5 (/dev/rdisk2) = d41d8cd98f00b204e9800998ecf8427e


I tried using /dev/rdisk1 and got the same results. I am OS X Lion.
If you are not trying to checksum disk the running system resides on (this won't ever be consistent) you may try


dd if=/dev/disk2 | md5 -