MacBook dual drive is killing hard disks

Towards the end of last year I decided to remove the CD drive on my late-2007 MacBook and replace it with a 2nd hard drive (using an aftermarket 'caddy').

Several months in, my 1 TB WD Blue (2nd) hard drive refused to open some files so I did a surface scan. This revealed the horror within! Hundreds of bad blocks, so I recovered as much data as I could and put the drive to one side. I replaced it with a different 1 TB Samsung drive. Last week the exact same problem has happened so I have removed the caddy and fitted the CD drive back in.

Is there something I have missed or does it look like the caddy is killing hard drives? It's either that or I have had major bad luck with hard drives.

Is there something I have done wrong? Or is it my laptop that is on its last legs? (this would worry me more). How can I tell?

Sorry if this sounds vague, but I'm quite worried. Please let me know if you need more information.

Try using one of the various temperature utilities to get the temperature of the hard drive. Perhaps the CD-ROM drive space isn't ventilated very well, and your hard drives have put out more heat than the original CD-ROM drive.