I have an USB HDD that I want to switch on and off with Arduino. I've done it with TIP120, it works, but the problem is, if I attach the ground pin of FTDI device to the Arduino's ground, the HDD turns on and can't be switched off. The voltage there on HDD is higher than normal (normal is when I switch using TIP120). It seems the current goes through this FTDI device rather than through the transistor. I think I know why it's happening. First, the USB HDD's positive wire goes directly to the power source (DC-DC booster). The negative wire goes to the TIP120's collector, but this negative wire seems to be connected also with the ground of the USB cable. This then goes to the USB HUB, then to the FTDI cable (I attach these 2 devices to the same USB HUB), then to Arduino, which in turn gets its power from the same adapter that I power USB HDD... So how do I solve this? If I don't connect the FTDI device's ground, everything works well. It also works if I don't connect the USB cable from HDD to USB HUB. (I need the FTDI to communicate with Arduino and to tell it when to turn the HDD on or off. I need TX, RX and ground for this, right?).
Here's the picture of the HDD USB-SATA chipset (HDD is 2.5', takes 5v, 1A):
(there used to be a DC socket. I removed it and attached the 2 wires. The + - that were on that DC socket seem to be connected to the + - of usb connector for some reason(?)).
This is the DC-DC booster:
http://www.ebay.com/itm/XL6009-LM2577-Regelbares-DC-DC-Booster-Modul-/301376967427?pt=LH_DefaultDomain_77&hash=item462b779303
This is the "diagram":
(where it says "problem!" it means the problem is when I attach the ground of FTDI to Arduino).
(I'm relatively new to electronics.)
