Behaviour of TM toward and extHDD vs TC?

I remember recently backing up my system to an external HDD using Time Machine, it worked very differently comparred to experiences of the past. In the past I had access to a Time Capsule, so I never thought about it, that's what the technology is for, but it died on me. So when I decided to use an external HDD I found that I could not just pause the backup, I had to let it run and because it was tethered by USB I couldn't use the MacBook because the connection was fragile and could be lost, the whole process took me over 24hrs!

I did some research and found that TM will not allow a backup to be resumed, although it will pause, when resuming TM just takes forever to carry on from the point it left that you'd be as well to just start again. My issue is that I plan to have one of the internall HDDs in a Mac Mini dedicated to TM, mirroring the other and so both of the same size i.e. 1TB.

How will TM behave to an internal HDD when backing up, it will be getting the data from an SSD, for use as my main drive.

Or, am I just imagining it?

Thanks.