External HDD doesn't show up in /dev on raspberry pi
Possible reasons why a USB hard drive doesn't show up in /dev when connected:
- Drivers for your USB chipset or hardware not loaded or not working properly (try updating kernel if possible)
- USB driver had a problem initializing the device - typically wrong driver, defective or unsupported hardware
- Some hardware may act bad if too much power is consumed by attached USB devices
- Hard drive is dead, defective or not communicating with the SATA/IDE to USB bridge (hard drive not connected to enclosure properly?) - SATA/IDE to USB bridge may also be dead or defective.
- Hard drive is not powered on or not getting enough power
- Physical issue with USB cable or port
Try putting the hard drive in an enclosure that is separately powered and see if you have better luck with that.
You are right 400ma is not enough for Raspberry to feed the external drive. I had a similar problem and solved it by attaching a powered USB hub, and voila!! disk recognized and mounted.