External Hard Drive stuck on 'Read Only'

For My birthday about 4 months ago, I received a 1.5TB Seagate external storage device. (I frequently do computer graphics/web design oriented projects, and have a lot of important irreplaceable files.) I only began using it recently but unfortunately found after putting only one file on it, the drive became stuck in a 'read only' format and I am disallowed from adding or deleting files on it. I do know that I that first file onto the drive I tested it by moving to another computer. (PC to Mac) so that might have something to do with it. As there is only one file on it that can be easily copied off and replaced, I have no concerns about wiping the drive if that is necessary.

Thanks in advance for your help.

Follow this link:

Solving read-only conditions for external hard drives - CNET