Slow Copy on WD My Passport 2TB

Hello
i have WD My Passport 2TB i bought it 2moth ago, when ever i use copy/paste in middle of copy/paste suddenly speed drop down and i it take too long to finish it. in some case it crash and stoped.
please help me should i take it back for guaranty or something?

USB 3.0 or 2.0 ? SDD or HDD? What speeds are you getting?