Can't Boot from Thunderbolt External Drive -- Please Help

[Dark Phoenix]

The subject says it all. Apologies if this subject has already been covered but I just spent the last 30 min looking for information on the site and have come up with nothing...

My Mac Pro (Late 2013) has a small internal drive of 256GB. I have Mac OS X installed on my external Buffalo MiniStation Thunderbolt SSD (Buffalo Mini) and would like to use that as my main drive to boot from. The issue is that even though I can see the Buffalo Mini under SYSTEM PREFERENCES > STARTUP DISK, it doesn't boot from the Buffalo Mini. Instead, it just boots from my internal drive

I've also held down ALT during start up and the Buffalo Mini not there at all to select, only my internal flash drive and the Recovery Volume. The Buffalo Mini is not even an option to boot from and yet, as I previously mentioned, it IS an option when I open System Preferences> Startup Disk in OS X.

But here's something to make it a bit weirder. I have a G-RAID Studio 8TB drive I use for Time Machine. Anyhow, a while back I cloned a form HDD on it. It's still there, I just haven't removed the files. In any event, when I connect this drive to my Mac, I CAN boot from the G-RAID drive...!

Can someone please help me out with this?
(And just in case you're wondering why I don't just use the G-RAID as the external bootable, since it's 8TB I figured this is best for backups/Time Machine.)

 

[rawweb]

Arg, 256gb in a Mac Pro these days is criminal. Hopefully Apple doesn't ever offer drive sizes like that agin if they ever decide to get their hands dirty and make a new one.

Without digging to deep into your problem, it sure sounds like for whatever reason that thunderbolt drive isn't boot friendly. From google search it looks like it's also a USB 3 drive, have you tried booting with just USB connected?

 

[g4cube]

Please make sure your external drive is formatted as Mac OS Extended or GUID partition format. This is required to have a bootable external drive. You will not be able to boot if MS-DOS, FAT32 or EXT3 partition format.