External storage drives are portable devices that connect to a computer (via USB, Thunderbolt, etc.) to add extra storage for backups, large files, or transferring data, offering portable capacity beyond the internal drive, with main types being slower but cheaper HDDs (spinning platters) and faster, pricier SSDs (flash memory). They provide convenient expansion and backup, but are susceptible to physical damage and accidental unplugging, with NAS (Network Attached Storage) offering multi-user network access as an alternative.