You cannot boot from a drive that is not GUID partition table formatted. Check with Disk Utility to see if you have them as MBR.
Are you saying GUID partition is required specifically for USB/external drives? The current partition type is likely Mac OS Extended (Journaled) if so.
It was prior the only internal drive in my 2011 iMac. I took it out of my iMac when I sold it and without changing anything on the drive, attached it via the USB 2.0 adapter that Samsung packaged with the SSD, powered up the Mini while holding the alt/option key and it did not list the SSD as a bootable disk.
I then picked up a SATA III to USB 3.0 adapter made by Roswell (SP?) and tried the same thing and again, the drive wasn't listed as a bootable option.
That's when I tried doing Internet recovery and in the OS install it does see the SSD and let me pick it as a drive to install to but when it does the reboot after the install it sites for a good 5 minutes trying to boot then I assume fails and moves onto the next bootable drive which is the OEM internal 1TB drive.
In ML on my OEM internal when I go to settings and start up disk it list the SSD and I can select it and reboot but again it'll sit with a grey screen and spinning wheel for about 5 minutes then (I assume) times out and goes to the OEM internal drive.
I borrowed a friends USB 3.0 dock yesterday and tried that and this one did show the SSD as an option when booting and holding alt/option but wouldn't boot from it so I again did the internet recovery and it installed, reboot and in this case sat forever at the grey screen with the spinning wheel and never went past that. I had to force power it off, disconnect the USB attached drive and power back up to get past the grey screen with spinning wheel and back into ML on the OEM drive.
I wouldn't of thought it would be so difficult to accomplish this. I did some test using Blackmagic and the internal OEM drive was doing 58MB/s Write and 66MB/s Read. I then attached the SSD via USB 2.0 and tested with Blackmagic and was getting 38MB/s Write and 39MB/s read. Lastly I tried the SSD attached via USB 3.0 and was getting 181MB/s Write and 200MB/s Read.
I realize something like thunderbolt would be over double the USB 3.0 speeds from what I've seen other on here post but I would be ok with what the USB 3.0 speeds can accomplish since it's still a good bit faster than the internal and wouldn't require me ripping apart my brand new Mini and running the risk of damaging it.