What cable do I need for Thunderbolt to Firewire?

So I spent $40 bucks on cables and ran into an unexpected problem when I started up in Target disk mode. I received a message that I had to upgrade my old computer to 10.7 before I can use my new computer as a target disk. I have Yosemite on my new Mac but only 10.6.8 on both my old laptop and old Mini. OS 10.7 requires 2 gigs ram, which neither of my old computers have, so I won't be able to upgrade them. Any ideas?

My computer is less than a year old and having irreparable disk issues when I am using the system diagnostic. Seems like it should be covered under warranty - should I just take it to the Apple store? I read somewhere that you should just get them to replace the hard disk if you have a disk error more than once.