Bulk-Only Transport, often abbreviated BOT, is the original USB mass storage protocol used to transfer commands and data between a computer and a storage device such as a flash drive or external hard drive. It has been supported since the early days of USB storage and remains extremely widely compatible.
The tradeoff of BOT is that it processes one command at a time in a fairly strict sequence, which limits how efficiently it can use a fast connection. The newer UASP protocol improves on this with command queuing, giving noticeably better performance on capable SSDs, though BOT remains the safe fallback that almost every device and operating system understands.
Most USB flash drives still use BOT, since their memory is usually not fast enough for the difference to matter, while performance oriented external SSDs increasingly favour UASP when the whole chain of drive, enclosure and computer supports it. See our external SSD guide for practical guidance.
