> We tried switch off 5V line (using a relay), usually works, but once we
> fried a camera.

Connectors are designed to connect shield > power > data in that order.
It could be that voltage on D+ and D-, after V+ had been removed, got
to the unpowered logic and damaged it. A similar thing can happen with
PICs and other devices. A PIC without Vcc can be powered through its
I/O pins, the current going threough the protection diodes to the Vcc rail
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