I think I would create one bootable FAT partition on the SSD and flash the Raspbian image into that partition. I would then create a non-bootable NTFS partition that I would keep my files in. I think when I plugged the drive into a running copy of Windows, it would ignore the bootable partition and just show me the NTFS partition. Then, when I put it in my Pi, it would boot into Raspbian using the boot partition and I could mount the NTFS partition to see all my media files.