Quick question about transferring files

[Soogs]

Wasn't sure where to post this so here goes

I need to transfer most of the files from my nas drive to my usb drive and vise versa

Am i better off doing it one way then the other or simultaneous

Im doing it via a mac

I guess what im worried about if fragmentation of files if i do it simultaneously

I seem to get better speed if i do it at the same time

Any input on this?

Cheers