importing photos with preview to external hard drive takes up space on laptop!!

I was recently trying to back up the photos from my iPhone 6.

I have an external hard drive and i use the function from Preview app "import from your iPhone" to export photos and videos to either the laptop or directly to external hard drive.

So, i did the same this time, i directly export the photos to the external hard drive to save space (i had about 12GB left on my rMBP late 2013 edition).

I was copying maybe 2000 photos and some videos and was copying copying until i realized it was chewing up my storage space on my rMBP!

I copied directly to the external hard drive but it was also taking up invisible space on my laptop which i don't know why.

I checked iPhotos app and unchecked the "copy items to the photo library" and resumed copying the rest of the photos about 500 of them.

And seems it fixed the problem, it did not chew up anymore storage on my laptop.

But now suddenly 10GB of space is gone and i only have 2GB space on my laptop and i can't find the reason why?

Did it create some temp files? or what?

the storage thing on MacBooks are a pain..............

Please help!!