Strange, but I have seen a similar issue a while ago... have to admit I am not a guru by any stretch but that sounds like you may have an issue with the PC drive space.
I used to get the issue mainly with HD Movies or Folders with shitloads of pics when i backed up a years worth.
Some "backup programs" need enough room on the original drive for a "temp" copy of the file they are handling.
By that I mean to say on your original drive you need enough room to have the original file and also a copy that the software buffers to "copy" across to your target drive (the backup)
That means if you have 10GB space and try and copy 20GB folder the process shits itself even tho you have say 100GB of space on the target.
I have a 128GB solid state drive that is for Windows only and a 2nd 500GB HDD that is my working PC.
I back up to 1 TB external drives no worries but when my PC drive used to get too full (when it was a smaller drive and I was lazy) I had to start with small folders until I had enough room for the buffering.
May not be your issue but worth a look see possibly
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