Do I need a desktop (3.5-inch) or portable (2.5-inch) external HDD?

Reviewers frame this as a tradeoff. Powered desktop units like the WD My Book and Seagate Expansion deliver far more capacity per dollar and faster sustained speeds, but require a wall outlet and are bulky. Portable 2.5-inch drives such as the Seagate Portable are bus-powered, pocketable, and travel-friendly, but cap out at lower capacities and run slower. Pick desktop for backups and media libraries, portable for on-the-go storage.