So that drive is off-site and so I haven't been able to look at it thuroughly but in order to have the opportunity I decided to instead append the Thursday backup onto my Tuesday drive (allowing me to have some downtime with Thursdays drive).
The Thursday differential that is now backing up on Tuesday's drive is running MUCH faster, and more in line with what I would expect (taking only 16-18 hours it looks like) which I think supports the idea that the issue is with the drive itself. Now that I have a week to play with it I have the time to format the drive and I hope this will solve the issue.
Thanks for the responses, let me know if you think I am going the wrong way with this.