Seagate Go Flex has a wire,
(USB)-----------------(SATA)
The USB side goes into your computer and the SATA side can be inserted into a generic Hard disk or the enclosed harddisk given by seagate.
*cache.gawkerassets.com/assets/images/4/2010/05/goflex25_leftangle_fw800_hi.jpg
This pic is not the USB version, but you can see the sata port.
That SATA port can be attached to the SATA port on
*www.arrowcomputers.com.au/wp-content/uploads/2011/08/DRINOTSATA1.jpg
So you can use the general HDD like external ones. Might not be useful for everyone.
I think that the 2.5" version can only connect to 2.5" drives and SSDs. 3.5" inch version can connect to all drives.