Why can't one simply dd an ISO file to flashdrive under OpenBSD?

Under Linux, I can simply do a:

dd if=ubuntu-14.04.1-desktop-amd64.iso of=/dev/sdb

and boot up the /dev/sdb which is a device for a flash drive. It finishes in 263 seconds.

But: If I boot up an OpenBSD 5.6 and do the same (for the same flash drive, zeroed it between the two tests):

dd if=ubuntu-14.04.1-desktop-amd64.iso of=/dev/rsd1i

It takes about half an hour to finish (using securelevel 1), but when I boot up the flash drive from it the BIOS only says:

no active partition found

Question: what am I missing?