I used to find it had something to do with the explorer thumbnailer finishing up but sort of not letting go. It would happen if I had pictures or videos on the USB drive, and if I got the error I could go to another folder like my documents, drag a picture into another folder, go look at the pretty new thumbnail, then I could remove the USB drive because the thumbnailer was ‘parked’ back on the C drive. Sounds like I’m making it up but I swear it worked.