Internal Drive A uses 10 out of 14 TB
External Backup Drive B uses 9 out of 14TB

Both contain Folder 1 through 4, identical names, identical contents except for the new contents that were added to Internal Drive A throughout timeIntervall between backups.

I use Windows 10 File Explorer to copy Folders 1 to 4 from internal to external backup. Windows then says "not enough space on (external backup drive).

Windows assumes that all 10TB present on internal drive A are needed on external backup drive B in the form of free space. OFC, since it doesnt know how much data isnt required to be copied over.

Is there any windows internal or third party option to alleviate this problem?