I've copy/pasted about 30GB of songs from iTunes onto a 64GB SDXC card. My new Boxster will identify only about 5GB of music (the first 5GB in artist alphabetical order as listed in my iTunes library.) Any idea why the car won't see the rest of my music? I've tried this on two different SD cards, and two different sets of music.
Additional, the files in iTunes include MPEG audio files, AAC audio files, and "purchased AAC audio files."
I think I know your issue: formatting.
I'm willing to bet that you have the SD Card formatted in FAT32, which has a file limit size of 4GB, give or take. While none of your individual 'files' are more than 4GB, FAT32 can have issues with large amounts of data in a single folder -- even if there are folders inside folders -- which is how iTunes does it.
The 1,000-file limit of PCM4 that @hkginlax
notes may have something to do with it, too. The solution is to delete the iTunes 'root' folder -- the one that holds the individual folders for each artist. (These days, it's usually called 'iTunes Music', I believe). Copy ONLY those individual folders, one by one (or, better yet: use 'Select All') to your card.
My SD card is formatted in exFAT. I've got about 110GB on it. The car reads it perfectly. Right now all of my music is in FLAC format, but I initially loaded AAC files to it when I first got my car; worked great then, too.
The one thing you can't do is use AIFF files -- Apple's proprietary lossless codec. It's one of the few that PCM4's DAC won't read.
Hope this helps!