If you are having trouble with your Garmin USB device, the garmin_usb
kernel module may have grabbed it.

According to http://www.gpsbabel.org/os/Linux_Hotplug.html, you can
prevent that by following these instructions:

According to the documentation at gpsbabel:
http://www.gpsbabel.org/os/Linux_Hotplug.html

To keep the OS from loading the (frequently dysfunctional) kernel
garmin_gps module, edit /etc/modprobe.d/blacklist and add the lines 

# stop garmin_gps serial from loading for USB garmin devices
blacklist garmin_gps

To allow the USB devices to be read and written by a non-privileged
user, create a named /etc/udev/rules.d/51-garmin.rules with the following
contents:

SYSFS{idVendor}=="091e", SYSFS{idProduct}=="0003", MODE="666"