SSD not being recognised in machine (though older HD is) but is working fine when booted from usb

[boycom]

Hey y'all, so something really weird has happened.
A week a go I cloned my SSD onto an external Hard Drive, and updated the clone from Mountain Lion to El Capitan. This was so that I could check everything worked smoothly on El Capitan before committing to upgrading my internal SSD, and everything was working fine. A buddy of mine wanted to do the same so I opened it up and at the same time had to give him about 8GB worth of logic files. Because it was reading off an external HD it was being super sluggish and froze so I force-restarted it. Upon trying to restart it as usual (from the internal SSD not the ext HD) I got a crossed out circle.

To test whether or not it was the cable connecting my SSD to the computer or if it was a corrupted SSD itself I removed it and replaced it with the HD that came with the machine.
Here's what I don't get:
The original HD powers up fine when in the machine so it's not the cable.
BUT the SSD when plugged into a SATA to USB connector comes up on my buddy's computer AND I can start it up using it (holding alt at the start and selecting my SSD) as the main OS which means the SSD ain't broken and is reading fine too!

SO, we have a machine that works fine and an SSD that reads fine, but when the two are put together, they don't get along. I'm currently trying to reinstall Mountain Lion on it using recovery, and that's taking 3 hours to download.

This is such a stupidly specific question and I don't really know what to do.

Any advice would be very much appreciated

Thanks

Noah

 

[chabig]

You said you have an external clone of your SSD. Just restore that to the SSD and you should be good to go.

 

[boycom]

You said you have an external clone of your SSD. Just restore that to the SSD and you should be good to go.

Yeah I thought that but that would mean being forced into El Capitan early.
Also, earlier today I conceded to that, and realised my buddy's MacBook Pro could not power both the SSD (going through the USB/SATA connector) and the external HD with the clone on it at the same time, so I can't even do that

 

[Weaselboy]

Still sounds like a bad internal drive cable to me. I've seen this often in the forums where a HDD works fine but a SSD does not with the same internal cable. My theory is that the higher data transfer rates of the SSD expose flaws in a borderline cable. The fact the SSD works okay in the external enclosure really proves it is in the internal cable.

 

[simonsi]

Replace the cable.

 

[chipchen]

I'm guessing you have a MacBook Pro 13" circa 2010-2012. The SATA cable on these guys are known to fail. You need to replace the cable and all will be fine.