External Hard Drive

External hard drives come in two core form factors, portable and desktop. Portable external hard drives are great in that they're usually small and light, and can be powered by the host computer. They are limited however by capacity however, as single hard drive portables top out at 2TB. In rare cases two HDDs may be put in RAID0 to get to 4TB in a portable, but currently there are only two products on the market that are taking this path.

Desktop external hard drives on the other hand offer huge capacity options thanks to 6TB hard drives, in some cases direct attached storage options can scale to several bays, netting nearly 50TB of storage available via external solution. Of course even single external desktop hard drives must be AC powered, so their portability is limited.

Both portable and desktop external hard drives connect via several interfaces, though the primary are USB 3.0 and Thunderbolt. The latter is more often used for Mac-centric products or for SSD-based external storage that can take advantage of the high-speed interface. 

Recent reviews:

Portable external drives
Desktop external drives