Recently I bought myself a Raspberry Pi 2. One of the first things I wanted to do was to install Kodi. This was easy enough thanks to the apt package manager included with Rasbian. It's as simple as sudo apt-get install kodi.
However since the Kodi user data and cache can grow quite large I decided to place the .kodi folder on a USB drive. This was as simple as copying the .kodi folder from ~/.kodi to my USB drive mounted at /mnt/USB and creating a symlink in /home/pi to the USB using ln -s /mnt/USB/.kodi /home/pi/.kodi.
The above worked no problem at all however I quickly ran into problems because by default my USB drive was mounted as root. Sure I could chmod 777 the entire directories but this didn't seem like the best way to go about things. A quick search online took me to the following SuperUser post. Within this post there was a nice one liner that allowed me to mount the USB with the user pi.
mount -t deviceFileFormat -o umask=filePermissons,gid=ownerGroupID,uid=ownerID /device /mountpoint
I simply had to use the id command to find the uid and guid of the user pi (id pi) and then place the command in /etc/rc.local to mount the device on boot, although I should probably move it to /etc/fstab.