How do I troubleshoot missing waterfalls?

I have experienced recurrent errors with Ubuntu stations and a RTLSDR
(after about a week or so of good working). The client was working nicely but I got no waterfall or sound.

I made the test rtl_sdr to see if it returned something as in:
rtl_sdr -f 144.8M -g 48 -s 250000 -n 1000000 iq.dat

As it returned nothing, I deduced the problem was near the USB port
(however lsusb was returning correct results).
So I rebooted the whole USB system (using /sys/bus/pci/drivers/…) and all the problems were gone.