WD ES MyBook 500 gb hd partition not recognized

I have a WD ES MyBook 500 gb external drive and yesterday while connecting it to a friends computer, one of the two partitions are not showing up in XP (also not in Vista). Disk Management shows that the drive is there and is healthy, although its not accessible and not showing it's name. More than that, Disk Management shows that the drive is completely empty and and file system is not recognized. But if I open the drive through a file recovery program, such as Power Data Recovery, I can easily access the drive and also recover all the data.
So that I could do, but it's 400 gb partition full of data and I have no place to recover it to. Also, this is not the first time that this has happened on this hard drive, but I want to figure out what's the problem without going through the whole time consuming recovery process.
So can anyone have an idea, what's the thing is about. Is is corrupted partition table (that's my idea, although how can it become corrupted all of a sudden?), MBR problem or what.
Also, on some computers this hard drive, even before that error, was showing up, but on some not.

Anyway, is there some easy way to recover the drive without actually copying all the data to another drive?