External SSD via USB won't boot by default

What enclosure are you using for the SSD?

Can you provide a link?

If the SSD comes with an enclosure provide a link to that,

How did you clone to the SSD? What app did you use?

- Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

About NVRAM and PRAM

- Try starting in Safe Mode

OS X: What is Safe Boot, Safe Mode?

- Boot to Recovery and repair the startup disk

OS X: About OS X Recovery - Apple Support

- If it is repairable reinstall the OSX. Nothing should be lost

How to reinstall OS X on your Mac - Apple Support

- If you do not have a backup use disk utility to restore the internal disk to an external disk so you can try to recover data.

Then format the startup disk and do a fresh install of OSX

Are other USB devices connected?

Is the SSD directly connected to the Mac?

I suspect the enclosure. Try another enclosure as a test. A USB 2 enclosure would be fine for a test

I would trade that $30 one in for a couple of the $15 Sabrent 3.0 enclosures.

I have three different Sabrent 3.0 model enclosure's from that Sac. Fry's and my 2012 Mini can boot from any of them. > http://www.frys.com/Sabrent+USB+3.0+enclosures

Good to hear that the SllG works. You're welcome.

hi Xiantez, I'm in the same boat as you! I'm glad that getting a new enclosure helped out; it's the only thing that I haven't tried yet (short of getting a new Hard Drive).

I was sold a NexStar 6Gwhen I bought my SSD a few months ago and it's been a struggle to get my SSD to boot off of the drive. Cloning my drive only made the SSD discoverable, but my Late MM2014 doesn't seem to care about that during boot-up. I'm going to look into at getting the Sabrent Ultra Slim 2.5 to USB 3.0 as my replacement case.

Does anyone know if it'll be an issue with my Samsung Evo 850?

Just be VERY careful when purchasing from this retailer. As a former long time business customer, we purchased some very expensive audio equipment, and when the representative was asked if the products had been previously sold and returned, he responded that they had not been. We discovered differently after unboxing the equipment. With a $30 enclosure that's not much of a concern but, nonetheless, just be careful.

????

I was not suggesting that the OP buy an open-box or return items...?

.

Apparently my implications that we purchased new audio equipment from Fry's but received returned/used equipment weren't clear enough. However, since we're straying slightly off the subject, I'll rephrase my comments for better understanding and more brevity.

Based upon repeated, documented, negative buying experiences, I suggest avoiding Fry's Electronics. Should Apple users choose to purchase new devices from that vendor, expecting to receive items that had never been returned, inspect the packaging very closely especially for devices you may be connecting to your Apples. Take photos immediately upon delivery of packages received from Fry's Electronics prior to opening if any damage or broken manufacturer's' seals are observed.