USB Attached SCSI (UAS) is a mass storage protocol that Microsoft added in Windows 8. It improves performance over the older Bulk Only Transport protocol with lower overhead and parallel commands. Windows uses the Uaspstor.sys driver when a device reports UAS. Windows falls back to Usbstor.sys when hardware streams cause problems.
Your external disk shows as slow or as a plain USB storage device. Microsoft documents the protocol behind the faster mode.
Facts follow the Microsoft Learn USB in Windows FAQ. UASP is the common name for UAS support.
What is the USB Attached SCSI Protocol?
Microsoft calls UAS a mass storage protocol designed to improve performance over Bulk Only Transport (BOT). UAS reduces protocol overhead and adds SATA native command queuing plus parallel command processing. It relies on a USB 3.0 feature called streams.
Which Windows Versions Use UAS?
Microsoft lists UAS as supported on Windows 8 and Windows Server 2012. Uaspstor.sys is new to Windows 8 and is not present in earlier versions.
What Hardware is Needed for UAS?
- Hardware streams. Needs a SuperSpeed USB device connected to an xHCI host controller.
- Software streams. Uaspstor.sys also includes software streams. High speed devices work on any host type.
When is BOT Used Instead?
Windows uses the Uaspstor.sys driver when a connected device reports UAS. Known issues with hardware streams on a specific xHCI controller or with a device UAS implementation make Windows fall back to BOT and the Usbstor.sys driver.
How to Check Which Driver is Active?
- Open Device Manager and find the external disk.
- Open its Properties and select the Driver tab.
- Select Driver Details and look for Uaspstor.sys or Usbstor.sys.
Microsoft does not describe this check on the FAQ page and the steps are general Windows practice. Windows shows Connected to USB 3.0 for SuperSpeed devices but that message does not prove UAS. Microsoft also states the UAS driver can suspend an idle storage device. Related reading: how to fix an inaccessible Outlook data file and how to fix the ZIP extract path too long error.
Frequently Asked Questions
What is UASP?
UASP is the common name for USB Attached SCSI support. Microsoft documents it as the UAS mass storage protocol.
Which Windows Version Introduced UAS?
Windows 8 introduced the Uaspstor.sys driver.
Is UAS Faster Than BOT?
Microsoft describes UAS as designed to improve performance over BOT with less overhead and parallel commands.
Why is My Disk Using Usbstor.sys?
Windows falls back to BOT when hardware streams or the device UAS implementation have known issues.
Key Takeaways
- UAS is the USB mass storage protocol added in Windows 8.
- Uaspstor.sys is the UAS driver.
- Streams give UAS its speed advantage.
- Windows falls back to Usbstor.sys when needed.
- Device Manager shows the active driver.
Sources and References
Facts follow Microsoft Learn.
- USB in Windows FAQ (Microsoft Learn) learn.microsoft.com