Hello I have an iMac retina 27 (since 10 months ago) and since I have El Capitan (Im not 100% sure but i didn't have these issues at the beginning) I have many problems when copying big files (from 2-3 GB) from/to external drives. I thought it was my old external 1TB HDD that was dying but now I have a brand new Seagate 2TB and it happens the same, also if I copy from an SD card from a video camera ir happens the same. The problem is sometimes it just gets stucked (and stops copying) and you have the dialog opened copying and at the end I have to restart Finder. I am trying also a file commander "Commander One" that seemed to fix it but today it is having the same :-( Any ideas what can it be? I haven't found anything around... When I download a big file a extract it in my internal SDD I have detected no issues.