exFAT external HD not readable

I have a new iMac (well, new to me) and I tried plugging my external hd (via usb), to it and it's not readable. It shows up in Disk Utility, but I can't access it. I don't want to reformat it, because I'll lose all my files. I had thought about installing Windows (shudder) on my iMac just so I can transfer the files... but I really don't want to. Is there any third party software I can use??

Specs:
iMac (24" Early 2008)
3.06 GHz intel core 2 duo
4 GB 800 MHz DDR2 SDRAM
NVIDIA GeForce 8800 GS 512 MB

Launch Disk Utility and click on the drive then click the Erase tab (don't worry nothing will be erased). At the bottom of the screen a window a message should tell you how the drive is formatted (FAT 32, NTFS, etc.) What does that message say?

It says it's in MS-DOS (FAT). But I thought I formatted it to ex-FAT... crud.

OS X should be able to read FAT-formatted hard drives. I don't know why it cannot.

It may be best to take it to a Windows machine, copy the data off, reformat the hard drive exFAT and copy the data back.

exFAT is fully supported for read/write operations by OS X. It is the most capable file system which is cross-compatible between Windows and Mac without adding 3rd party drivers.