Try this:
1) Create a folder on your USB and call it for instance BackUp (move the WindowsImageBackup folder here or you can use the WindowsImageBackup folder
2) Right click the newly BackUp folder or the WindowsImageBackup folder if you didn't make a new folder
3) Click on properties
4) Click on the sharing tab
5) Click on the share button
6) In the file sharing dialogue box choose the current logged in user or everyone if you prefer
7) Then click share once you've made your choice
8) Once successful you will get a new dialogue box showing the location of the shared folder, right click it and click copy link
9) Now open the system image wizard
10) Paste the link of the share you created above (in the network location)
11) You should now be able to use your USB to back up going forward.
I am perplexed why Microsoft decided USBs were no longer valid locations for back ups...
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