Introducing Portable HDD
Portable hard drives, which are used for backup and extra storage in addition to transportation, are comparable in capacity to many internal hard drives, Portable drives offer more performance. The portable hard drive is powered by a USB connection instead of external drives meant for settled desktop use, which draw power from the wall. Due to its lack of moving parts, lower power consumption, and increased resistance to physical damage, portable SSDs are also quieter.
or,
Portable hard drives are a subcategory of external drives. All portable drives are external drives, but not all external drives are portable. They are designed to emulate the drives found in laptop PCs. Powered through a USB connection to a computer, they are built to be small, lightweight, and easy to carry.
Therefor