I'm currently trying to build a hardware USB-MIDI-Recorder based on an Arduino Leonardo. The idea is that you attach it to a self-powered device like an E-Piano that features a USB-B out port for sending MIDI packages and record those to an SD card.
It would be nice if I could actually power the recorder from the USB port of such a device directly so that I don't have to use an external power supply, however, I think those USB ports are only slave ports most of the time.
Is it still possible to draw some power from them or would that damage the device? This is probably out of specification, but is there still a chance? The recorder would probably draw less than 100 mA.