Error Opening Image File in Terminal
In order to access a raw disk device /dev/rdisk1s2, it should not be mounted. Open Disk Utility and unmount it first. Otherwise you would get a Permission denied error when you try to access it as a raw device.

If you wish to (or need to) have it mounted and use the block device, remove the "r" from the disk name, making it /dev/disk1s2 (notice the missing "r" before "disk1s2").