External HDD is read only?

So I just bought my mac mini with lion os. I had this external hdd with my pc desktop and I would save all of my movies, music, etc... to it so when I did decide to change over everything would be on the external and I wouldn't have to make a huge thing of it but after I got my mac mini all set up and the external set up I see that I have read only permission and there is no option to change it when I do a cmmd+i everything is greyed out. and it says REAd-ONLY at the bottom of the window. I have over 500GBs of info on this drive. How can I change it to write and read permission? I cannot add or change items on this at the moment and its quite annoying especially in itunes when I need to edit songs, names and albums...

It's formatted as an NTFS drive. Mac's can only read NTFS natively. There are some 3rd party applications that allow you to write to NTFS drives, but I personally don't use them.. so can't recommend them.. and to be honest, don't recall their names.

That said, this comes up regularly.. so if you search for NTFS, you'll be likely to find recommendations.

Purchase "Paragon NTFS" from here for $19.95. Highly recommended and it works well.