Hi Elizabeth,
I am Sumit here to assist you with this question.
The backup simply fails and is run when you plug in the external drive the next time. There is no temporary location. Because these backup tools are deprecated, I suggest using a 3rd party program:
Backup tools in Windows 10 are deprecated and are not being actively maintained. I recommend switching to a 3rd party solution like Reflect, which is more flexible and is reliable.
https://www.tenforums.com/tutorials/61026-backu...;
Content on the above blog is written by a Windows MVP so it is safe. However, do watch out for the ads.