#1
 Not sure where to go for help but I have moved locations and now my main recording rig is located elsewhere. I am left with my old 2 channel interface which is an M-Audio FastTrack C400. This little guy used to be so solid on windows 7 (5 years ago), but I am now on an iMac and it just disappears at least twice a day and the only way to get it to show back up is a system restart. Any ideas????? 
#2
Try flashing the drivers and getting latest firmware.

Maybe try a USB2 hub? I am thinking that it could be losing sync because of the newer USB3 ports on the Mac.
Found this:
https://forums.macrumors.com/threads/el-capitain-wont-recognize-my-m-audio-c400-interface.1925802/