Hi, im new to this forum. Haven't been able to find an answer for my problem, so now im giving all you hdd guru's a chance to shine

My problem is that i've been givin the job to fix a WD2500JB HDD for a friend. This HDD was inside a My Book Essential 250GB external HDD. It stopped working, and now i've split it apart to see if i can get it to work.
I thought i had a old WD HDD that was similiar to the WD2500JB one. I did have an old one, but was'nt entirely the same. I did however, try to put the old PCB on the WD2500JB. After that, the PC did try to regocnize the HDD, but it could'nt. At this point the HDD also started to make the clicking noises i've read about in here. So i came to the conclusion that it was probably a bad idea, although it gave me some idea that the problam had to be the PCB.
Long story short, i have to find a PCB that works on WD2500JB. How can i be sure that it works? What numbers/codes do i have to look after?
Cheers