timechine can backup another HD?

hello,

I use mini mac(Apple M2 2023), and two external HD.

I want to back up a external HD (A) into another external HD which I chosen as Time machine.

when I open Time machine option, I see that HD (A) on the list of 'excluding back up".

But I could not choose (-)... means I can not include HD(A), isn't it?

HD(A) is newer and 2TB bigger than HD(Time machine).

the size is the problem ? or...

all of my important data is in HD(A) not in my minimac at all..

is anybody know the solution of this problem?

thank you