Not sure this query belongs here; if not, I apologize.

I get a pop-up saying that the periodic automatic file backup cannot be completed for lack of space on the hard drive. I assume it refers to the D:\ disk, where my Data is saved and stored.
But thatís unlikely, because the tree shown by the Windows Explorer says there are only three main folders within D:, which together add up to less than 73.5 GB (far from the 300 GB capacity of that drive).
Those three main folders are: Program Files (x86), with 0,28 GB; Users, with 1,5 GB; WindowImageBackup, with 71,6 GB (after installing Windows 10). And the option to display the hidden elements is ticked up.

Note: This machine has also got two other drives: C:\ with 87,6 GB used out of a total of 284 GB (it contains the programs and applications installed, including Windows and, now, Windows.old), and E:\ with 11.1 GB used out of a total of 12.9 GB (the Recovery disk).
Thank you for any help.