If you’ve come across this already then this might be obvious, but in order to mount a USB external drive on an Ubuntu VirtualBox guest running on a Mac OS X host, you need to unmount the drive in Mac’s Finder first. Then using either click the USB icon in the status bar in your Ubuntu guest and you’ll see the drive un-greyed out (when it’s mounted on the Mac it appears greyed out and you can’t select it) – click it and it will mount automatically. Or you can do the same thing from the VirtualBox menu, Devices/USB.
MySQL install on Mac OS X
I haven’t used my MySQL install on Mac OS X for a while and I’d forgotten where it was located 
By default, it gets installed to /usr/local/mysql-versionnumber, with a symlink /usr/local/mysql pointing to this dir.
Mac OS X: Adding to your PATH
If you install an app that runs from the shell and you want to add the path to the executable to your PATH env var, just add it to PATH in a .bash_profile file in your user’s home dir – this will be run every time you start a new terminal session:
export PATH=$PATH:your_path_here
Mac OS X: Opening TextEdit from Terminal
open -a TextEdit filename
You can replace TextEdit with any app name to open from the shell