It may be the problem that occurs with a lot of USB powered devices jrherrer. Not enough power provided.
The USB cable plugged into the computer, provides signal wires for transferring information back, and forth, and also provides 5 Volts DC for power. (Two D cell flashlight batteries produce 3 Volts DC)http://en.wikipedia.org/wiki/USB
[ Below shows the Signal pins, and the Voltage pins of the USB B Mini style connector, (Male), used to connect to the SimpleDrive Mini external harddrive,
Pin 1 is for the 5 Volts DC Positive (Wire color - Red)
Pin 2 is for the D- Signal (Wire color - White)
Pin 3 is for the D+ Signal (Wire color - Green)
Pin 4 is not connected. (Or sometimes joined to Pin 5 with a Resistor)
Pin 5 is for the Ground (Wire color - Black)
Just added info in case you wanted to know ]
From what I see the SDM/250RW has a USB port on the side, and a DC Power Jack, used for an AC adapter, also.
(Round port with a pin in the center)http://www.hitachigst.com/tech/techlib.nsf/techdocs/0C876B7AB91B23988825773F006BEA02/$file/SD_Mini_DS_final.pdf
From what I see stated from Hitachi, and sellers on the 'net, the AC adapter may not be provided.
Many of those who provide a review state that both the USB cable, and the AC adapter is needed to provide enough power. Seems to be a power hungry external harddrive
It's a 2.5 Hitachi SATA laptop harddrive inside that external case. (The Harddrive is 2 and a half inches across in Width. Common size used for laptops)
When using an external device powered by a USB cable there are general guidelines to follow,
A) The USB cable should not be more than 2 feet in length
B) Connect the device directly to your computer, instead of through a hub, or other device, unless the USB hub has it's own power.
2) USB cable:
Could be a faulty USB cable, although I imagine you have tried different ones by now.
3) Interface Circuit Board:
The Hitachi SATA 2.5 harddrive inside the SImpleDrive Mini SDM/250RW, plugs into a small circuit board located inside that external case.
Components on the circuit board may be fried, or the Harddrive may have come loose from the interface.
Loose from the interface
The back of the harddrive has connector pins sticking out.
The interface has socket holes which the pins insert into.
Faulty circuit board
The external case of the SimpleDrive Mini SDM/250RW can be carefully opened up, and the Hitachi SATA 2.5 harddrive extracted out.
Another inexpensive external case for a 2.5 SATA harddrive can be procured, and the Hitachi harddrive put in it.
Example of a 2.5 SATA external enclosure,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3743049&CatId=2781