500mA is the same as any home computer puts out through the USB ports, and it will power any notebook hard drive. I've got a notebook drive plugged into one of my USB ports at home, no additional power needed, no problem. A 3.5 inch drive would definitely require external power.
According to the available manuals, FLAC is not supported, only AAC, MP3 and WMA.
As for the USB connections, the single DIN HUs have it on the back, double DIN on the front.