Files on Mac

I recently found that there was 350GB of files in a hidden directory on my iMac. The Directory was within my account and the title began WD…
Within it there some hundreds of files with began with the the name .dat.nosynch…all of 61.5Mbs
I deleted all the files and immediately freed up 350GB of space. I suspect that these files came from WDmirror. Can anybody through light on this?

I just discovered the same sort of thing, after my Mac started telling me my startup disk was full. I freed over 50GB of space, but within hours it disappeared, so I started looking - thought I must have had some malware.
I have spent a few hours searching for something about this with no luck, aside from your post. I am heartened to hear you deleted them with no apparent ill effects.
For me, on OS X 10.11.6, the files are under /Library/Application Support/WD Sync/[my drive name]