When trying to use pktriggercord with a Pentax K-r I was unable to get it mounting via USB. I'd get lines like this over and over in /var/log/syslog:
Feb 21 14:11:12 raspberrypi kernel: [ 996.673083] usb 1-1.2: new high-speed USB device number 49 using dwc_otg
Feb 21 14:11:13 raspberrypi kernel: [ 996.933126] usb 1-1.2: new high-speed USB device number 50 using dwc_otg
Feb 21 14:11:13 raspberrypi kernel: [ 997.193121] usb 1-1.2: new high-speed USB device number 51 using dwc_otg
When looking at other USB problems people had documented solutions for with the Pi I found that turning the USB down from hi-speed to full-speed helped in some cases. To do that edit /boot/cmdline.txt and add this to the end and reboot
dwc_otg.speed=1
That worked for me.