Can't copy files from one hard drive to another (same sizes)
Time machine uses Unix hard links. This is confusing the copy software which results in the same file having multiple copies. 

Here is how to make the copy according to Apple.

https://support.apple.com/en-us/HT202380

So, give your father the other drive.  I read somewhere where it's not best to mix in non-time machine files on a time machine partition. Seems reasonable to me.