Restore partition type to NTFS
ExFAT is not the same as NTFS, it is however cross-compatible with Mac (10.6.5+), Linux and Windows (may require additional drivers/software).
It seems as you have unintentionally formatted your NTFS-partition to an ExFAT partition, and depending on which version of OS X you're running your Mac can only read from that file system and not write. (Unless you have additional software installed to handle that file system)
You can try and reformat the partition in Terminal in case the Disk Utility is being silly, you will lose all files from that partition (they're technically not gone, but you'll need third-party software to restore them).
To format the partition open up the Terminal App and run:
sudo diskutil list

Find the partition that matches your USB-partition you wish to format. It should looks something likes this (This is from my own harddrive). Find the correct identifier for the ExFAT partition.

Now run the following in the Terminal:
sudo diskutil eraseVolume {FileSystem} {Name} /dev/diskN

Replace {FileSystem} with HFS+ or similar (only natively supported on Mac, not Windows/Linux) and {Name} with a name for the partition. Replace /dev/diskN with the identifier for your ExFAT-partition. Make sure you type the right identifier, if you type in the wrong one you could easily wipe your Mac's internal harddrive.
Now the partition should be reformatted, unless any errors occured. Now you can open up Disk Utility and choose to format the partition as NTFS if you don't want to stick with HFS+.
This might not be the most effective way to do it, I'm no expert so anyone with better knowledge feel free to come up with a more effective solution