Looking for a way to image an entire harddrive to an external usb drive

"dd is the best way to do this. While the method you posted will technically work (which not output file is specified, dd sends the output to shell). It would be better, as Wil says, to use
sudo dd if=/dev/hdX of=/dev/hdY
Not that the disks could also show up as /dev/sdX. If you're not sure which disk is which, type sudo fdisk /dev/hdX, and press p to print the partition table for that device.

dd does a bit-by-bit copy, it produces a forensically sound image, so any recoverable files will be there.

One of the best tools to mount the image on Windows that I've found is WinHex, it lets you browse as a normal disk, and has some data recovery features. However, GetDataBack (NTFS) has much better recovery, and it can also access image files."

"if you want to preserve the drive content for further forensic examination, i recommend EASEUS Disk Copy, a potent freeware providing sector-by-sector disk/partition clone regardless of your operating system.

and speaking of forensics, there's nothing better than WinHex, a 1 year personal license is pretty reasonable too (€39.41).

since it is a NTFS partition, there's no reason why you shouldn't be able to mount it in Windows, however, i would not recommend it, because Windows my write information to the drive which may make forensic examination even harder, use WinHex from a BartPE CD/USB stick instead to avoid any writing."