I see the same problem on both of my Windows computers after updating from 1803 to 1809. One machine uses a directly connected USB drive, the other backs up over the network to a NAS drive, but the failure is the same on each. It's clearly just a bug and one that needs to be fixed ASAP. It seems like 1809 shipped with a bunch of issues ... there was a Cumulative Update today (https://support.microsoft.com/en-us/help/4464330/windows-10-update-kb4464330), but this issue was not addressed in it.
I bugged this on Feedback Hub ... I encourage everyone affected by this bug to do the same.
link?