You're not alone in finding the iDrive a little infuriating - I too found it took quite some time getting used to. Once you get the hang of things, it does all work nicely though.
I discovered that the USB port in the glove box was fussy for some reason - I had 2 different USB flash drives - both 4GB, and both formatted identical, and both loaded with same exact music files, but the USB port only recognized one of them (it saw a SuperTalent USB drive, but did not recognize a Kensington branded one). I also used the same USB drive to download a BMW software update to my iDrive which was available from the BMW website. As someone pointed out earlier, the internal hard drive stores music files in USB1, USB2, etc. groupings for some reason, so when deleting, you would have to delete each group.
What really bugs me is why the USB port in the glove box cannot be used for connecting an iPhone or iPod, as I would rather have it hidden away there than sitting out in the open in the console.