I couldn't connect to / mount my Canon Rebel XS / EOS 1000D when connected to my computer's USB3 ports, but I could when plugging it into a USB2 port. When plugging into the USB3 I would get "PTP I/O Error". I don't know if this is a problem in gphoto or a problem in the Linux kernel, or what, but I figured I'd start here.
dmesg on USB3:
[ 3585.400774] usb 3-2: new high-speed USB device number 15 using xhci_hcd
[ 3585.420440] usb 3-2: ep 0x81 - rounding interval to 32768 microframes, ep desc says 0 microframes
[ 3585.420447] usb 3-2: ep 0x2 - rounding interval to 32768 microframes, ep desc says 0 microframes
dmesg on USB2:
[ 3614.263054] usb 1-1.2: new high-speed USB device number 11 using ehci_hcd
Linux 3.4.0-030400-generic #201205210521 SMP Mon May 21 09:22:02 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux