Updated now external boot HDD won't mount!

I have been running Mountain Lion from an external WD Passport USB 3.0 HDD as I am not rapt in it, just want to access iBook occassionally and other ML only software.

It had 10.8.2 installed and mounted and ran fine.

I applied the OS X Mountain Lion 10.8.2 Supplemental Update 2.0, assuming it probably was advisable for security and stability.

When I rebooted after the update my Passport hard drive won't boot/mount. It refuses to mount until I have started up from my internal HDD, pulled the USB cable and then reinserted it.

I have repaired the HDD with Disk Utility although nothing was reportedly wrong with it and other than multiple attempts at trying to restart and force it to boot by both using the Startup Drive in Systems Preferences and holding down the Option key, I am getting nowhere.

Has anyone any suggestions of anything else I can try?