Problem accessing a partition on an external hard drive

I'm wondering if anyone can help me out here; I have a strange situation.
I'm running Windows 8 and I have an external hard drive that won't let me access 900GB or so of its 1TB. The drive mounts about 10GB, and the rest is nowhere to be found. If I go into Disk Manager I can see both of them: the 10GB reads as K:, FAT32, healthy, active, and primary partition, while the other 900 reads as healthy and primary partition, no letter or anything else. If I right-click it, my only options are delete volume and help. It's brand new, and the 900GB has nothing on it.
Here's the catch: it's not my hard drive. This hard drive actually has to go back to a Mac user, NOT a Windows user, to reiterate. I've only got so long with this thing and I have to put some files on it. How can I make this 900GB usable for me so I can put files on it, and then usable for a Mac user?
I'll provide any information you need. Thanks!

the mac part is what got me to my answer, it is because that 900GB partition most likely in format mac os journaled and not seeable by windows at all and never will be it has to be fat 32, NTFS (which a mac can't write too without special drivers), or ExFAT which if you are sure that there is nothing on it then you can delete the whole partition setup of it and format the whole thing to ExFat as that is something mac and windows can read and write too.

As rvcjew said the unaccesible partition is most probably formatted in hfs+ (the mac default format). You can either try accessing it with 3rd party tools :

This might help you : Sharing Disks - Windows Products
I've only tried Catacombae - HFSExplorer and a long time ago, it seemed to work fine and it's free.

Or you can reformat it in a format accessible to both windows and mac. Mac can read NTFS but can't write to it, unless using 3rd party tools like NTFS-3G - Wikipedia, the free encyclopedia. Both OSes can read/write to FAT32, but this format is limited : max file size is 4GB, less efficient at storing data with big partitions (large cluster size). Or in exFAt as the poster above said which can be accessed with Mac OS X and Windows since XP.