Why is my SSD booting from USB and not from SATA

I installed OSX 10.10.4 on a Samsung 840 EVO using an external SATA - USB enclosure.

When restarted, my MBP booted from the SSD but when I installed the SSD in the HDD SATA bay all I got was a map picture with a question mark in it. The SSD was not recognised.

Then I put the HDD in the external enclosure and restarted. The MBP booted from the HDD and in Finder the SSD was present and could be explored.

In system preferences the SSD was marked as boot drive.

The SATA bus is 6 Gb, the cable is OK (working fine with the HDD) and I installed the latest firmware for the SSD.

Any ideas what can be wrong ?

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Did you first format the drive for Mac, make it partitioned GUID and MacExtended Journaled?

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Yes I did and I did a fresh OS X install

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You seem to have done everything correctly. It is a 6Gb/s bus. Boot from the external HDD and re-check the format on the SSD and see if you can run Disk Utilities. Next, that cable may be fine with the old, slower drive but may have issues with the faster drive. I'd go after the cable and replace with a 2012 wider data path cable.

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Thanks for your quick response and your advice.

I have 2 more questions :

Is the cable for the mid 2012 MBP compatible with the late 2011 version ? The product information only states the mid 2012 as compatible.

Is there also a possibility that the cable connections are not fitting well or that there is something wrong in the logic board ?

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These HD/IR cable are notoriously bad. They usually failed as they come over the last stair step and get hit by the bottom case. Yes the 2012 works, I've installed dozens. Sometimes you can only insert one screw. After I put the new cable in, I put pads on either side of it to prevent the bottom cover from hitting it. Apple started using pads in the 2012+. Be sure to order 13" as the logic board inserts are different on the 15" model.

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OK, I ordered a new 2012 cable, but what kind of pads should I put in ?

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