Here is one possibility, which assumes the hard drive you want to replicate is wd0. Note that I chose to use the raw (character) device, as I have found this is usually faster than cooked (buffered) device for this sort of thing.
If the number of sectors are evenly divisible by some blocksize, you can increase the speed of replication by using the "bs=" operand. Read the dd(1) man page.
# dd if=/dev/rwd0c of=/nt_1/myopenbsd.img
Here is another method, where the output is piped through a compression step:
# dd if=/dev/rwd0c | gzip > /nt_1/myopenbsd.img.gz
|