Hi l5b5,
We appreciate you reaching out to us. We understand that your videos freeze after moving the files to a separate hard drive.
Let's try to isolate the issue.
First, let’s check if the issue persists when signed in on a different user account.
The article below can help if you do not have any other user account to test with.
Set up users, guests, and groups on Mac
If you are experiencing the issue when signed in on a different user account, please check if the issue persists while on safe mode.
Safe mode does a basic check of your startup disk, similar to using First Aid in Disk Utility.
Performing this step may help resolve the issue, or further isolate the issue.
Below is an article to help guide you through the process.
How to use safe mode on your Mac
If possible, try to move a file back to your Mac mini's internal storage.
This step will help further isolate the issue.
Take care!