I just had a similar problem with the Promise SmartStor DS4600 that looked like a dead power supply issue but was "something else" more stupid and unexpected.
I unplugged the apparently dead DS4600 to check the power supply (link to the manual with replacement instructions -->
http://www.xander.com.hk/product/product_manual/prod_manual_221.pdf).
Flipping the device upside down, through the vents I could see a green light on the power supply was on. Thinking this was odd, I waited a few minutes for the light to go out.
I then replugged the DS4600 in and it worked fine.
Check for this in case it's a common issue.
As a general comment, I've found this to be a very sensitive device and it will shut itself off for no apparent reason. However, this is the first time I ever had an issue like this.