Error 0x10003 (No Free Space) Error/Solution make no sense in context.

I ran into an issue today trying to backup data from an 8Tb external HDD to an identical one. When I try to run a backup of selected files/folders, the program throws error 0x10003 (details below).
I checked the Knowledge Base and found the relevant troubleshooting guide.
However, neither the Error/Solution makes sense in the context. I am not migrating an OS, there is no SSD involved, and I am not a laptop user.
Do I just need to shrink the partition on my HDD (as if it was an SSD) that I want to backup to, or is there another solution?

Full Error Message:
Description: Scheduled Backup Failed
Reason: No Free Space (cannot create partition of requested size)
Error Source: Paragon Backup and Recovery 2017
Error Code: 0x10003