Hello xasmanius,
Welcome to Apple Support Communities!
If we understand your post correctly, you've recently copied some files from one USB device to another and after doing so the system drive was full and you want to know if your Mac is saving a copy of these files where would these files be found. We'd like to help!
We recommend using Spotlight on your Mac to locate the files you're looking for: Search with Spotlight on Mac
Below is an article that will help you to narrow down the search results: Narrow your search results on Mac
Let us know if you have any questions.
Thank you for using Apple Support Communities.
Take care!