As gary implied, an EHD is not a backup since programs you move to it are no longer on the Hopper. You've transferred the problem from one disk to another.
That said, you can back up an EHD with any PC O/S that understands the ext3 file system. Or you could make an identical copy of sectors (which makes the file system irrelevant) to an identical disk. In principle you could do the latter with the internal disk as well, or the former if (as I have read somewhere) your O/S understands the xfs file system.