Need a little more info on what you have and your plans. BUT as a starter let me outline what you would do IF you have the following situation:
1. This is your second drive, and you already have a boot drive you use with your OS installed on it.
2. This is a new drive with nothing on it you want to save, and no Partitions already established.
In that scenario, all the tools you need are in your Windows system. Alternatively, you may be able to download from your HDD maker's website a small utility that will do this work for you.
My plan here is to establish the first Partition on this drive as the small one with the FAT32 File System. Mainly that's because I do not know whether a PS3 can handle HDD units and volumes over 128 GB (technically, whether a PS3 supports 48-bit LBA). Then the second Partition will use all the rest of the space. I will assume you will connect the external drive to your computer to create the Partitions, etc.
The main tool you need is Disk Management. Click on Start in the lower left and in that menu, RIGHT-click My Computer then choose "Manage" from the mini-menu. This opens a new window. In its left pane expand "Storage" if necessary and click on "Disk Management". This opens on the right two panes, each of which scrolls to show you all its contents. The upper one shows you all the disks Windows is able to use now. The LOWER RIGHT pane has a series of large horizontal blocks, each representing a hardware device. For each main block there will be a small label sub-block to the left with a name like "DISK_0", a size, and some other info. To its right there may be one or more blocks representing Partitions already established on this device. (The optical drive main block will have no sub-blocks because you can't establish Partitions on it.) For example, your boot device will have a big sub-block showing its letter name, "C:", its size, its File System, and some more info. If there is more than one Partition on a unit they will show as separate sub-blocks. If there is some space on the drive unit not already assigned to a Partition, it will be "Unallocated Space".
Look for your new 1.5 TB unit. If it really has nothing on it, it will be all Unallocated. If not - that is, if there is one Partition (or more) on it, stop now and consider: do you want to save what's there? If yes, you will have to do that before proceeding, because my procedure next is to destroy anything on this drive! If there are only Partition(s) on the drive that you DO NOT want to save, then RIGHT-click on it and choose to Delete the Partition. If there are other Partitions remaining, Delete them all, one at a time.
Now you should have a truly empty 1.5 TB unit. RIGHT-click on the Unallocated Space and choose to Create a Partition. This should be a Primary Partition and, since it's only for storage and not an Operating system, it does NOT need to be bootable. Set its size manually as you wish (about 100 GB). The next part depends a bit on what OS you're using. Some will have you complete this Partition Creation step now and then return for Formatting (by RIGHT-clicking again on the Partition you just created). Others will have popped up a wizard as you started and will include in it the formatting option selections. Either way, the two main options you specify are the File System to install - in this first case, you want FAT32 - and a Quick Format or Full Format. Quick Format does the basics; Full Format does that and then exhaustively tests EVERY sector of this disk for errors, and that can chew up hours of time. If you want to be cautious and have spare time, set it to Full and leave it run.
I suggest at this point after Formatting the first Partition you exit back out of Disk Management and reboot the machine so Windows Registry can catch up on what you've done. My Computer will show you the new disk with its own letter name, but you still will not see there any hint of the rest of this large 1.5 TB unit. Now you go back into Disk Management and get to that Lower Right pane again. Once more, RIGHT-click on the remaining Unallocated Space and choose to Create a Partition. You can make up to four Primary Partitions on one disk device, and many more Extended partitions. Since you want to use all remaining space in this second Partition, it also can be made Primary and not bootable. But this time you set its space to be all that is remaining (about 1.2 TB), and in the Format section you choose the NTFS file system. Make the Partition, Format it, exit back out and reboot again.
You should have two new "drives" in My Computer, each of which is really one of the Partitions, and each of which has its own letter name and is used as if it were a completely separate "drive", because it really is as far a Windows in concerned.
Now, IF your plan is to use this 1,5 TB unit as your only drive, or at least your boot drive, the actions will change a bit. Similarly, if you have data on the drive already that you want to preserve, that's a different story. If you have these complications, do not follow that outline. Post back here for more precise advice.
Odd label, but exFAT must be the right choice for the small 100 GB Partition to be used by the PS3. FAT32 is only the last version of a sequence. FAT16 came before for hard drives, and there also was FAT12 for floppy disks. Early systems just called them FAT, because the expansions had not happened yet.