I'd still think it's a power issue. I've got a bunch of 500gig laptop drives, and ended up getting a 10A 5v supply with a powered hub. Also if you have the chance, power the rpi by the 5v GPIO pins rather than USB, as often the PMIC on the Rpi is anemic and loves to STILL drop under recommended voltage. I run 5.2v 5A PSU on the 5v rail, and haven't had issues.
If these are 2.5" HDDs, (laptop sized) then maybe not. If they're the full sized 3.5" HDDs, they need their own external PSU.