I was working on a video recorder product that could record videos directly to iPods, iPhones and other media players. iPods were formatted to HFS+, if you connected them to a Mac out-of-box, and were formatted to FAT32, if you connected them to a PC. The platform i was working on was an RTOS, and we had to develop both FAT32, and HFS+ from scratch. It was not read-only, it supported both read/write.