Large external HDD for backup - Partition? Image and Synctoy?

It is because of the LBA and 32 bit counting you get with mbr.

In theory you can have a starting sector at 2 tib.

Then you have nowhere to go. The partition will have no size - it can't count any higher.

Anything above 2tib cannot be addressed using mbr

It might be better to think of it as a start sector anywhere up to 2 tib and a max disk size of 2tib.

Gpt uses 64 bit counting - that is how you can get bigger disks recognised.

The MBR partition table is a database restricted to 32 bit precision. The OS certainly is not. So from the MBR database you can start with a partition starting point with 32 bit precision (2TB) then an offset with 32 bit precision (2 TB) within this partition.