Most portable hard drives have smaller (recording) platters and slower rotation speeds, and thus you get slower read and write times, vs regular PC drive read and write times. 

If you need more storage than portable drives currently offer, 10 TB at most, you can buy multiple portable hard drives or buy a single desktop drive (which are also faster).

Good portable hard drives support USB 3.0 but are backward compatible with USB 2.0 versions. They also have decent warranty and provide solid customer support.

The best portable hard drive: 2TB Seagate Backup Plus Slim (compact, fast, reliable, cheap)

The second best portable hard drive: 2TB Seagate Expansion (same quality as Seagate Backup Plus Slim, but bukier)

The best portable hard drive with more storage: 4TB Seagate Backup Plus Portable (less cost per terabyte than other models)

The best portable hard drive upgrade: Samsung Portable SSD T3 (five time sfatser than others, solid state drive, pricier)