USB Attached SCSI (UAS) Mass Storage Device is a protocol used to improve data transfer speeds between a USB device (such as an external hard drive or solid-state drive) and a computer. UAS is an advanced protocol compared to the traditional USB Mass Storage Class (MSC) and is typically used with USB 3.0 and later devices, offering faster performance and lower latency. The UAS protocol is often used in devices like external SSDs for improved data throughput.