Clearing Copy to External Drive Cache that Makes StartUp Disc Full - Where to find it / how to delete it?

My start up disk was getting full on my Mac Pro (Late 2013 Quad Core, I have installed 64 GB 1866 MGz DD3 RAM), running OX El Capitan with 250 GB HD - it was completely full and constantly buzzing warnings etc - so I found I had used 192.85GB on my user folder - I have deleted my desktop, downloads and cleared trash. I appropriately did some Google research on how to move user folders and application folders to an external drive using a sudo ditto command.

I created a new user account and have tried to copy files over using terminal (my files are time machine backed up). I get error messages in Terminal as it churns through files noting that start up disk is full and no space for copy cache (or something to that effect). So I terminated the terminal window.

I cleared another 10GB by deleting some applications on my HD that I can re-download.

I tried the same process, and got the same error messages as Terminal ran the copy command and terminated the terminal window again. When I search both accounts, clear Trash, run MacKeeper I cannot find where the 10GB of cache is stored, or how to remove it, and I wonder how many GB of cache I have.

Any tips on how to find / clear this interrupted copy cache to free up some space, and secondly, any alternative ways to move or copy a large wad of files (192.85GB) to an external drive without filling up cache on the source drive?

Many thanks in advance,

Michael