Format the drive in FAT32 - this is simple on the Mac.
Personally I would not transfer the iTunes music - even if it isn't in a compressed format. I have found issues with ripping music via iTunes.
The best solution is to rip your music collection with dedicated software. And in flac format.
The best software to use is dbpoweramp, but this is only Windows compatible. You can use XLD on the Mac, but it is arduous by comparison.
Kyri