Hi Peter,

Unexpected Windows shutdown may cause to hard disc problems. One of such problems is Error "Windows Explorer has stopped working."
If you received this error, you cannot do any hard drive operations like copy, past, cut, delete etc.

The cause of this error is bad sectors. Data is stored on your hard drive in sectors and the ones that the windows is attempting to save to have gone bad over time.

To solve this issue, you need to run ‘check disk’ activity.

1. Open your computer (My computer: Win key + E).
2. Right click on affected volume (Example drive D) and take ‘Properties’.
3. Go to ‘Tools’ tab. 
4. First tool is to ‘Error-checking’. Click on ‘Check now’ button.
5. From given small window, click on ‘Start’ button to start scan. (Ensure ‘Automatically fix file system errors’ option is checked).
6. If you check ‘Scan for and attempt recovery of bad sectors’ you might wait much time to complete scanning. If scanning without this option is not solving your errors, recheck drive after checking (tick) this option.
7. Sometimes you will get a message with ‘Windows can’t check the disk while it’s use’. Here you have to close all things related to this drive. Or click on ‘Force a dismount’.
8. You may need to wait some minutes to complete checking according to your drive capacity. After successful checking and fixing, you will get a confirmation message of scanning and fixing drive errors.

This is the common method to solve this error in Windows.

I hope this helps in copying files to USB.

Regards,
Lokesh