I'm completely clueless, so bare with me ... I just switched over to a mac. I have an external hard drive that I used with my PC and I can read it on my mac, but I can't modify it in any way. Is there a way to format it to my mac without losing everything on it?

Well, most likely the drive was formatted under the NTFS filesystem, for which Mac OS X has read-only capability. You can use the third party ntfs-3g driver to gain write access to your NTFS volume, but beware: it's still in beta and if you value your data I'd recommend having it backed up somewhere. That said, here is the howto:
http://www.lifehack.org/articles/lifehack/how-to-read-and-write-ntfs-windows-partition-on-mac-os-x.html

If you're going to be using this drive heavily under Mac OS X, I'd recommend backing up the files and reformatting the drive to HFS+, as this is a lot safer under OS X. (The downside of this is that you'll need special Windows drivers in order to access this from the PC platform if it's formatted in HFS+. Sadly, there seems to be no good filesystem for which both operating systems can natively read/write to.)

thanks! :)

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.