"Lost" my HD

Hi,
I bought a new 500 Giga WD MyBook USB external HD. Since it’s sold formatted for Mac, I had to repartition and reformat it for my WinXP Pro (SP3) system.
I can’t explain what exactly happeded, but eventually I found my system in a very queer state:

  1. The new HD got the letter drive of one of my internal hard drives (D. In the Computer Management it appears with a red bar and a “Healthy” description
  2. One of my old hard drives shows as an “Unallocated” drive, with a black bar and no letter assigned.

Looks like I’ve lost all my data on this unallocated drive!

Now, I would like to recover these data and hope to do it with the assistance of Stellar Phoenix Windows Data Recovery application.

Should I first partition this drive? Will this repartitioning let me reassign a letter to the drive enabling data recovery or will it lead to a total loss of the data?

How do I get out of this maze?