Thank you for posting here, @ML74.
I'm glad to share with you the places where you can save your backup file from the external drive to your computer.
First is, you can create a specific folder in your computer then transfer your backup copy in there. This way, you can locate and restore the file easily.
Also, you can save it on your computer's C drive. This serves as the default location for the QuickBooks Desktop (QBDT) company files. To do so, go to C:\Users\Public\Public Documents\Intuit\QuickBooks\Company Files.
To learn more, please see this article: Back up your QuickBooks Desktop company file.
For your future reference, you may view this link: Move or reinstall QuickBooks Desktop to another computer. This will provide you steps to move your QuickBooks Desktop from old to the new computer using a Migrator Tool.
Feel free to post here if you have further questions. I'll be around to help. Take care always.