Please help, hard drive won't mount.

I have an IMac OSX 10.3.9. I have an auxiliary USB 160gb LaCie(Seagate) hard drive which is nearly full. I made the mistake of trying to plug it into a Windows 7 64bit PC. It wanted to reformat it, which I declined. I plugged it back into my IMac and now the IMac wants to do the same. The drive will not mount but does show up in the disk utility.

Does anyone know how I can get the hard drive to mount again? If I need to purchase software to fix it, which is easiest to use and least inexpensive?

Thank you for the help.

When starting your computer pull up disk utility by the install disc and you will want to repair the HDD then verify it. I had the same issue and this fixed it.