WD Passport 1TB USB 3 - Very slow for USB 3... even after Format!

I have run CHKDSK on the WD Passport and it has found the bad sectors and marked them. Now the Passport appears to work at good USB 3 speed! The problem is that by marking specific parts as bad sectors the space has been reduced for some MBs. But my main concern is that I don’t know why the bad sectors appeared and I am not sure if those are due to software fault or physically damaged sectors for some strange reason (since I never badly treated my drive, or it never fell out of my hands to the ground!..). Anyway, the problem is that I really need the drive to store my movies which are now at my internal HDD drive, to save some considerable space (1TB !), so I don’t know if I could wait for weeks for the RMA to take place, since I am not sure if the store will replace the hard drive right away with a new one for me.

So, I am really not sure if I should buy and try the HDD Regenerator to see if this can completely fix all my Passport’s bad sectors, or if I should just try to return the drive to see how fast I could get a replacement (I believe this is still within warranty).