Can Windows tell me what is using my USB drive?

You can use Sysinternals Process Explorer to find the handle for any files that are open. Just select the Find menu and select Find Handle or DLL. In the dialog that opens enter the drive letter into the search box. The search results should show all of the files that are open from the drive and which process has them open.

"You can also use command line to query the Windows log, with wevtutil.exe (since Windows 7) with the knowledge that the Windows Kernel-PnP uses Event ID 225 to log system (always having process id 4) refusal to remove or eject the device USB\VID_####&PID_############ (where the #'s denote hexadecimal numbers).

wevtutil qe System /q:""*[System[(EventID=225)]]"" /c:5 /f:text /rd:true

qe System : query events from System log
/q : query with XPath
EventID=225 means the system refused an ejection request
/c:5 : number of entries to retrieve (5 here)
/f:text : format (default is xml)
/rd:true : reverse order (newest first)

I use it in a batch script."

"You can start resmon.exe (through WIN+R), go to disk > Disk Activity > Sort by File
Now you can see all files being accessed by the system and which processes are accessing them, ordered by the file path (which btw starts with a drive letter).
May not work with all cases, but it's a simple approach.

Restarting the computer seems ""free up"" device usage.
Also for faster removal, you can disable windows caching on Hardware section of your device, sometimes windows will take longer than expected to flush the cache to external disk and will display that message saying that the device is in use (because it will be, by Windows itself)"

"If you open ""My Computer"" and your drive is not listed under the ""Removable Storage"" headers, then Windows is viewing it as a fixed system resource for some reason. You will have to unmount any partitions on the drive.

If this is the case, open ""Computer Management"", then go to ""Disk Management"". For each partition on the device, right-click the partition, select ""Change Drive Letters and Paths"", and remove any drive letters assigned to that partition. Once you do so, you should find that the ""safely eject"" feature works as you had hoped."

"Had USB that windows reported as being in use...same as everyone else here.

In Windows 10 Ctrl+Alt+Del gets to Task Manager.

Find by scrolling - Windows Explorer. and highlight.

Bottom right is a nice friendly button called ""Restart Process""

Double triple check that ""Windows Explorer"" is the only thing highlighted.

Mouse left-click the friendly button ""Restart Process"".

I only had a single Explorer, others sometimes have two. Just note which one comes back on. Try and close/eject the USB. Mine worked fine, close the USB.

If yours does not eject/close, then probably it was the other Explorer. Try again and Restat that one. Good Luck."