If you're really worried about it, grab a free Linux dvd from a random computer magazine. Ctrl+A, then delete everything on the drive. Empty the recycle bin. Now copy and paste the dvd onto the drive as many times as it takes to fill it completely. When done, Ctrl+A again, delete and empty recycle bin again.
If you've got an agreed RMA, they're not going to go a deep dive file recovery, they'll be logging the serial number in a database and chucking the drive in the recycle pile. Emptying, filling and emptying again is more than enough.
If you're really worried about it, grab a free Linux dvd from a random computer magazine. Ctrl+A, then delete everything on the drive. Empty the recycle bin. Now copy and paste the dvd onto the drive as many times as it takes to fill it completely. When done, Ctrl+A again, delete and empty recycle bin again.
If you've got an agreed RMA, they're not going to go a deep dive file recovery, they'll be logging the serial number in a database and chucking the drive in the recycle pile. Emptying, filling and emptying again is more than enough.