My Data Copy Too Slow

Hi, I don know why  my EXT HDD suddenly drop down speed only like 100kb- 1mb ±,

i ask the computer shop from sold it to me, he just ask me to backup it and send back to WD,
but my file is more than 900GB, how long i need to backup it? 

Any solution to do it? Thank You…

Before you backup your files, follow the link below for the steps to run a test with the DLD tool.