How to format unallocated space

Hi!

I'm new with Mac OS X and I have an external USB 1 TB HDD that I want to use with Time Machine. This hdd had 2 partitions 500 GB each, formatted in NTFS, because I used with Windows. While my new iMac was comming home, I decided to get ready to use the external hdd so I repartitioned the HDD. I erased first partition in order to format it later with the iMac, and I formatted the second partition with exFAT.

Now that I have my new iMac and the external hdd connected, I can access partition exFAT without problems, but I cannot format the unallocated space with Disk Util, what's more, I can't do anything on this, except repartition the whole disk. Do you know why?

I know I can connect the hdd to a Windows machine, format the space with whichever format and then in Mac OS X, erase and reformat in HFS+, but why??