I have Raspbian installed on my Pi. I want to create a FAT32 partition on the unallocated space on my SD card. I followed this guide to create a partition. The partition is readable/writable on Raspbian and Mac OS without any problems. However, the partition does not show up in Windows Explorer. I opened Disk Management on Windows and found that this is no drive letter assigned to that partition. And Disk Management does not allow me to assign a drive letter to it.
Windows doesn't know to look past the first partition on an SD card, so won't find the one you made.
Here is a ext2+ driver for windows. good luck