[xDrive50i]

Hard drives are not supported and not recommended for a few reasons but mainly because of power draw. Most USB powered hard disk drives require more than 500mA to spin-up and or operate. The USB specifications state that 500mA (@ 5V = 2.5W) is the maximum a device is allowed to draw in high power mode. It also depends on the maximum power the USB port in the car will actually supply.

Other things you should keep in mind is the heat buildup in the compartment that might lead to premature failure of the hard drive and if your hdd is in a case which it most likely would be, the temperature inside will be considerably higher than in the compartment! most drives will start to give you trouble above 50 degC which is easily reached in hot climates or on hot summer days in the compartment with the hdd powered!

Another issue is vibration and/or shock. Even though most of these drives are specifically designed for portable equipment, they still don't like shocks or vibrations and again this can lead to premature failure.

I use a 64GB USB stick, it uses less space and power, is faster in terms of access and smaller and easier for me to take along (not that I really do that but if I wanted to update my collection, it's easier than having to lug around a USB HDD).
I ripped all my CDs onto my home media server and stored them in lossless mp4 format (14000+ songs) which I then convert into 256kb mp4s for storage on the USB stick. The 64GB stick holds just over 7000 songs. You can also create playlists and store them on your memory device and the car will pick them up and play them for you!