Macbook Pro can open files from Touro ext. hard drive but can't save to it. Solutions without data loss?

Hello,

I've looked around for a solution to my problem but I haven't found any specific solutions and I'm not comfortable in trying general ones for fear of loosing my data. Before moving I backed up all my document and music files from my PC to my Touro external hard drive. My main goal for buying this hard drive was to use it to store my itunes music library to save space on my computer and transfer my music from said PC tp my new MacBook Pro after the move. Today I set about doing so and was able to set my itunes up to pulll from the hard drive as well as load all the music I had saved on the hard drive into my MacBook itunes. The trouble came when I tried to download new purchases. It turned out I couldn't because itunes was unable to save them in the new file location on the Touro. I checked into it and I have the same problem with the documents as well; I can open anything already saved on the drive but I can't save any new files. It keeps on telling me I don't have permission to make edits. I'm afraid this mean I have to reformat the Touro for Mac but doesn't that mean I'll lose all that data? Does anyone know of any somlutions that would give me access to the drive without compromising what's already on it? I would just plug it back into the PC and see if I could adjust sharing settings, but it's all the way across the country now. Any help in this matter would be greatly appreciated!

Thanks,

-G-