What we often do in partition is run Disk Management, the default tool programed by Microsoft; however it's not the absolute solution, nor the ultimate solution on all partition issues. the following issue is not a fan of Disk Management.
Run Disk Management, right click on system partition, you'll find that Extend Volume grayed out, because there's no access of extend volume in FAT32 table, it's only works on NTFS (New Technology File System). You need to convert FAT32 to NTFS if you want to use Extend Volume in Disk Management. But this is not the purpose of this passage, which means to share how to extend volume in FAT32 system partition.
You can run Macrorit Partition Expert, you'll see the disk map listed. Click on the system partition, and choose Resize/Move Volume on the left column or right click on the system partition in the map, in the popup window you can do the extension: type in particular size or just use the handle to adjust. Each way is acceptable.
If there's not, we'll make one from other partitions
Run Disk Partition Expert, choose a partition with large size, anywhere is OK, no matter it's a neighbor partition or not, we have solutions. Situation One: the unallocated space is right behind system partition, which may not often happen, use the omnipotent Resize/Move Volume function, and directly soak up the space,that's the fastest way of enlarging C partition.
Situation two: unallocated space is at the end the disk, the very end of it, severed by partitions. Again we use the almighty Resize/Move Volume command, if it is at the bottom of the disk, move it to the top, run Disk Partition Expert, move nearby partition to the end on seeing the mouse turning into a cross-arrow, and do the same job if you have many partitions on the way. Don't worry; the whole operation won't take long. Stop moving when the unallocated space becomes the right-side neighbor of system partition. Then again we use Resize/Move Volume on C partition to absorb the free space.
Thus, you have a larger C partition in FAT32 file system when Disk Management doesn't work, and you can create FAT32 Partition if you have any rest unallocated space haven't extended by C drive
FAT: File Allocation Table
Fat32 max volume size: 2TB, or 16TB for 4 kb sectors
Fat32 Max file size:4GB
Above are the Basic data of daily use. You can find more on Wikipedia