External drive won’t read on PC after plugging into MAC (GPT Protective Partition)

The issue is simple:

Drives prepared with either intel-based MACs (OS X 10.4.6 and newer) or Windows XP 64bit, Server 2003 64bit, 2003 SP1(all versions), Vista and Windows 7…all these operating systems will prepare a drive using a technology called GUID Partition Table (GPT) which was created to allow the use of drives larger than 2 terabytes. 

GPT is not compatible with systems other than what I listed above, which is why you can’t see your drive when you plug it into a machine running say Windows XP 32bit. 

Great, so how can I see my data?

You can’t…not on the system you’re currently using.  Use one of the systems listed above. 

or..if you don’t care about losing the data…you can clear the drive to be used on your 32bit XP system.

Here are instructions on how to do that: http://knowledge.seagate.com/articles/en_US/FAQ/207837en?language=en_US