PC external hard drive to iMac?

I don't know if this was covered or not, but I couldn't find anything on it. I have an external hard drive that is formatted to my PC. I am able to read files on the hard drive with no problems, but I can't write to it. Now, is there a way to format it to Mac without taking everything off and putting it back on?

A friend of mine suggested installing MacFUSE. I tried that and it still won't let me drag and drop into the external hard drive. I am using this mainly for pictures and videos that I take on a daily basis. I want to be able to just move them over there at any time.
What am I doing wrong?

Sorry, this is my first Mac.

Well, if you don't plan on using your other computer (I'm assuming Windows?) with the external anymore, then you can just partition the remaining space on the external and format it in Mac OS Journaled. That way you can set up Time Machine to do daily (hourly, weekly, whatever) backups to the external and you can select which folders to backup.

I don't want to use it as an automatic backup and it won't be connected to the Mac all the time. I want to manually put pics and files on it and then I put it in my fire safe at home.
I don't have the PC anymore, so if I can just reformat it, that would be best.