My Book - Read Only

I was using my MY Book drive to backup my PC Laptop, but now I have cnnected it to me Mac it is read only and I can seem to find where to free it up to read/write

A drive formatted for Windows NTFS is read only on a Mac. Try NTFS-3G on the Mac Make sure the data on your external isn’t the only copy you have. Some people have had the drive become corrupt and unreadable after using on Windows and mac.