memory leak when writing

Hi all, I have a simple 2 disk mirror with new 8tb drives connected by sata. I have had good performance with it. Just tried to perform a routine operation, copying ~500GB of video. Within 5 mins every app started to beachball as all the ram was exhausted and a large swap file built up. I stopped the copy, rebooted to clear the memory and repeated it with no problems. Hmmm. Any ideas? Much appreciated.