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Summary: Use Windows PowerShell 4.0 function to find USB drives on Windows 8.1.
How can I use Windows PowerShell to find how many drives attached to my system are using USB interfaces?
Open Windows PowerShell 4.0 in Windows 8.1 with admin rights, use the Get-Disk function to find all disk objects, and then filter for a bustype of USB:
get-disk | where bustype -eq 'usb'