After playing around with a thumb drive, I think I have some of it figured out. I have only tried this in my '12 RX350, and I will be doing this next weekend with my wife's 2010 IS 250 C. When I first tried the the thumb (USB) drive, it would not work for me. Come to find out, it was formatted for NTFS, and it needs to be FAT32. So I took all the folders (Albums) and copied them to a temporary folder on my computer, refomatted the thumb drive, then I copied the entire contents of my temporary folder back to the thumb drive. When I loaded the thumb drive in my RX, all the folders were in order as they were on my computer (alphabetical).
Two days ago, I added more folders to the thumb drive, but my RX did not see the new folders in their proper alphabetical order, but in the order I copied the folders on to the thumb drive. That tells me if I take a little time to organize my music on my computer then transfer it all at once to the thumb drive, the folders (albums) will be in alphabetical order.
If you want to have them sorted by the order of the albums you want to hear, then copy the folders, one at a time, to the thumb drive in the order you want to hear the albums.
I tend to like my albums in aplphabetical order, that is how I kept my records, LPs, 8-tracks, cassettes, and CDs. I do not own an "i"anything, but I spent 8 bucks for a 16GIG thumb drive that will hold over 100 albums and I am happy.