I am using a mid-2010 Mac Mini running OSX 10.11.6 as a "headless" music server. It is connected to a DAC, via USB, and thence my stereo system. I use iTunes, playing mostly rips of CDs. If I power the DAC off and then back on, the Mini reboots. The DAC has multiple input channels (TOSlink etc) and the problem occurs even if it's switched to one of the non-USB inputs. I am at a loss. I have tried resetting the SMC and NVRAM. I have tried different USB ports, as well as a USB isolator (between the port and the cable to the DAC). I don't believe it's a kernel panic, because I don't get the "your Mac restarted because of a problem" message. I don't believe it's the DAC drawing too much power (I need to try a USB hub to test this) because the DAC mfg'er says the USB input draws very little power and because I don't get the usual error message (when you plug in a USB device that draws too much power for the port). Thanks for any ideas !