When I had issues with my portapack, it took me some time to remark that these issues only happened with a particular USB cable.
The SDR would work fine, and then when I would try to record or playback, the screen would turn dark.
You can see this in the following video:
What is happening, is that this particular USB cable is electrically defective: the voltage drop is too large, due to the abnormally high resistance of the cable. The portapack doesn’t receive enough power, and starts to malfunction.
In the following 2 videos, I perform various tests with that defective cable:
Videos on my video blog (with some info on the devices I used):
Didier Stevens 
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