exFAT works on Windows (XP, Vista, 7 and 8) and OS X (10.6.5 and newer), so it should work without any problem. If you format it on OS X, a Windows PC should be able to write on it without any problem.
Note that, before erasing the external disk, you have to copy the files onto the internal disk, so you won't lose the files
For any and all reading this topic, it should be made known that MS very recently dropped all further development of exFAT. It was never finished as it was (a final specification was never released).
If you need to share a drive between platforms, then the best supported options are:
1) Format the shared drive as NTFS and install Paragon NTFS on your Mac.
2) Format the shared drive as HFS+ (Mac OS Extended) and install HFS+ in Windows.
I have tried the first, and it works very well. There's also Tuxera NTFS for Mac, but Paragon's software is cheaper and does the same thing. Can't comment on how well the Windows software for Mac formatted drives work.
I recently formatted a 16GB drive on Mountain Lion as exFAT.
It was NOT readable by a Windows XP machine, but was happily readable on a Windows 7 machine (or was it Windows 8? Not sure).
No idea of Vista's behavior.
exFAT has to be formatted on the Windows machine and XP needs exFAT installed as a free download from Microsoft.
Don't buy the F.U.D.
There would be tanks in Redmond if Microsoft even attempted to discontinue support for a drive format they released.
With NTFS one just has to keep paying over and over as Microsoft makes changes to it for licence fees they collect from companies using it.
A truely open drive format is needed, but with two corporations playing games for profit by controling hardware and software it's kind of hard to implement.
Why Apple formats exFAT in a manner that can't be easily read by Windows remains a mystery.
Kurt, what is your reference for saying that MS has dropped development for exFAT?
It was another member of these forums. Normally very accurate in their statements, so I assumed them to be correct. I looked again myself and found out this is not the case. At least, not exactly.
What MS did was make a change in Windows 8 so that you cannot format an internal drive as exFAT. You can still format any type of external drive that way (hard drive, USB drives). The whole purpose of exFAT was to allow easy transfer of large files between platforms in a format that didn't require third party drivers on either the Mac or Windows to read the drive.
To make sense of why MS would cut off the use of exFAT for internal hard drives, it has to be understood that exFAT still doesn't have a final specification. In other words, it's still a work in progress. So Microsoft's thinking is probably something like this; "Don't use exFAT on an internal drive. Use NTFS, which is known to be stable and reliable. It's okay to use exFAT on external drives since you can get the original data again from an NTFS drive if the external exFAT drive is having trouble."