Trying to back up a failing drive

I have a Hitachi 2 Tb drive in a MacPro 5,1. The Hitachi drive is failing.

The Hitachi drive has about 180 Gb of information on it. It has Mojave installed.

I also have two, 1 Tb Samsung EVO 860 SSD's in the MacPro.

The first EVO 860 has Catalina installed. I partitioned that drive into two 500 Gb volumes. The first 500 Gb volume, in addition to Catalina, also has the TechTools Pro 13 Edrive on it.

The other EVO 860 drive has Windows 10 home installed on it.

What steps - step by step - should I use to use TechTool Pro to back up this failing drive to the open 500 Gb partition?

Thanks for your help.