The Unix utility dd is a disk copying utility that can be used from the command line to create a disk image. It can make a bit copy of the drive it's copying. If you have a drive that spins up, but does not mount because of damage, you can use the following:

dd bs=512 if=/dev/disk1 of=/some_dir/image.dmg conv=noerror,sync

Using the switch noerror allows the copy to ignore the errors, and adding mode sync fills the image with nulls at that point.

The bs=512 selects the block size (in this case 512), and if=/dev/disk1 is the Unix path to the actual disk device. Make sure the chosen directory (some_dir) has enough space free to take the entire disk image. You will end up with a disk image the size of the source drive's maximum capacity.