External hard drive problem

So my brothers external hard drive isnt working... please help
in our windows xp pc it doesnt show up in my computer. shows up in disk managment but if you right click all the options are unclickable. plus I wasnt able to asign a letter to it. it says it is empty now too. so he lost everything. well no biggie we just want to be able to use it again! I plugged it into my mac and it showed up and I dragged something in and it seemed to work fine but now the pc says its empty. anyway I really have no idea what to do. been searching the forums and all but the disk gives me no options when I right click on it (where it shows up above and down below). so I dont know what to do!
 
When in Disk Management, on the left hand side where you see "Disk 0", "Disk 1", "Disk 2" etc, does it say that the drive is "Offline" ? This can cause all the options to be greyed out.

If the drive is showing as Offline then simply Right click the Disk that shows as offline (In that same box that says offline) and select Online. This should allow the drive to be shown in My Computer like normal.

EDIT: After re-reading it, I noticed you said you assigned it a drive letter and it now shows up but it is empty? The hard drive may be having issue's, check into if it is showing up as Offline or not and post back.
 
hmmm no it doesnt say it is offline!
other things I forgot to mention is the computer that he had it hooked up to before had a serious virus that I removed so I think it might be infected as well.
also under status in the disk management it says "partition protective GPT"
dont know what that means but ya.
ya the drive letter is blank! I went to reasign it but it is still blank!

thank you so much for your help
 
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