Error copying files to external drive – “There is not enough space on DRIVE. You need additional XX GB to copy these files” (Windows 10)

Context: was using windows 8.1. Every week, I back up work files (about 51 Gb) on an external hard drive (note: that collection of work file is also my OneDrive folder). Been doing it for years without ever encountering a problem

Problem: I upgraded to Windows 10 and now, whenever I try to back up my folder on my external drive (1.5TB of free space), I get the following error message

“There is not enough space on Documents. You need an additional 53GB to copy these files"

Notice that the space cited in the error message is for the originating drive (i.e. where the OneDrive folder resides) andnot the destination drive!

I was able to copy several of the sub-folders individually. But for most of them, I received similar messages.