Hi Mate,
I've gone through this thread and couldn't find the solution I'm about to post. Sorry if someone did and I missed it.
Simply, if you get this error, all you need to do is format the external drive to Mac OSX Extended Journaled. This will stop the error. This really is a better file format for macs and much more compatible with them.
Most external drives some formatted to ExFAT these days. This allows them to work both for Mac and Windows. However, this isn't optimal for Macs. So, if you have an external drive that you just bought from somewhere and you want to use if as a backup drive, that will only be used on a mac, then it's a good idea to re format it to Mac OSX Extended Journaled. NOTE, formatting the drive will delete it's contents and by formatting to Extended Journaled, it won't work for Windows anymore. Keep that in mind. It should however, fix your issue.
Here's the apple thread on formatting external drives on Mac OS's Format external drives to Mac OS Extended before using with Aperture - Apple Support
Steve Morton
http://digitaliron.com.au/