[St. Lou Stu]

The error
"There is not enough space on Users. You need an additional x GB to copy these files."
is a free space error.
When you initiate a large transfer, the first portion of time is spent checking free space, which is why this error may not immediately pop up. Once the files are counted and free space requirements calculated, it then looks to the target machine to see if there is enough space. If not, you get an error.

When you transfer single or smaller batches of files the free space calculations happen much faster and appear almost non-existent. Small transfers are succeeding because you haven't yet reached the capacity of the destination drive.
I imagine that if you were to transfer all one-by-one they would eventually start failing due to space limitations.

You really don't want all of windows.old anyway. It has a ton of extraneous ****.