Time Machine in OS10.6.8 problem

I'm on a late 2007 (2,1) Mac Pro. Running OS10.6.8

I was using Time Machine quite successfully with no problems until fairly recently when my TM Hard drive got full. I decided to change the TM HDD to a bigger one (both are external FW800). The drive WAS at the end of a 3 FW daisy chain. Since then I've tried moving it to the front FW800 port. But I'm still having the same problem:

When I run TM I very soon get the following message and TM stops:

"TM could not complete the backup. Unable to complet backup. An error occurred while creating the backup folder."

I'm tried trashing my TM preferences (In Mac HD/Library/Pref). Same issue.

I've also run Disk warrior on my prgram drive and fixed permissions issues. On running Disk Warrior "File check" there are issues that WILL NOT repair: 2 weird looking plists (from Malware maybe?) for which it says "Detected that the Property List data is damaged and cannot be repaired." I cannot delete these since it's in a root folder which I don't have permission to access for some reason....

Can anybody tell me:

1. Why time Machine is not working?
2. Are these 2 weird Plists something to do with it, and regardless, are they bad?...

Thanks!