Hard Disk problems, unsure of how to proceed A couple of days ago, I closed the lid on my 2012 MBP running El Capitan with Samsung Pro 830; it had nearly no power left. Power ran out while I was closing the lid, I suspect, so I plugged it in, and it wouldn't come back up. So I powered it on. It wouldn't then boot.

Tried NVRAM and SMC resets; no success.

Startup key combos:
Tried starting up in safe mode; no success.
Tried startup manager; no success.
Tried Diagnostics; no problem reported.
Tried Single user mode; no success.
Tried verbose mode - got EFI error.  

Swapped in another disk to try and restore from an SL DVD from my older MBP; no success (apparently you can't install an older version of OSX than that it shipped with).
Put knackered disk back in, tried System Restore - success! This got me into Disk Util for ML (original OS for the 2012 MBP) where I could repair the disk. Diskutil found some problems with the EFI, and it booted into My ElCap OS, everything seemed ok. Ran diskutil again for good measure, no problems reported.

I turned it off so that I could secure the hard disk and the bottom cover of the machine.

It then wouldn't boot again.

I then tried internet recovery and got an error with an exclamation mark over the little globe on the restore screen with an error 'apple.com/support: -2105F'

This was now a little beyond me, and Google wasn't helping, so I phoned Apple. They suggested that I try a new disk, as mine was completely bajanxed.

So then I got my 2010 MBP (also running ElCap) and downloaded the 'Install El Cap' program from App store with a view to creating a local boot USB drive to attempt recovery.

I have borrowed a (120Gb) SSD from work, formatted with a GUID partition type and OSX journaled FS, and plugged in and booted from USB drive.

The install process nearly completes and then fails, not really giving me any error info.

Tried this a few times, same results, reformatting new SSD on second MBP, also via USB El Cap Diskutils. No problems showing on the new SSD.

Internet restore also fails on new SSD (with the same '-2105F' error.)

The only thing I've got going for me right now is that I managed to mount the broken boot disk onto my 2010 MBP and decrypt the volume, figuring that one less obstacle can only help (took 8 1/2 hours, but completed successfully!). I can read the files on this drive, so I haven't lost data per se. Once decrypt was done, I ran repair_packages from terminal on 2010 MBP on the mounted (broken boot) disk - it found some errors and repaired them. fsk_hfs also completed fine with green lights.

The last attempt at El Cap install completed successfully (tried whilst writing this), and it restarted - white screen, with the no entry symbol. The strange thing was that I didn't get the 'traditional' pop up with the 4 choices when trying to install ElCap, just went straight into install.

Wondering whether there was a setting saved somewhere, have now reset NVRAM and SMC again, and trying install. The pop-up menu has now appeared, and am trying again.

Has anyone got any suggestions, as I'm running low. I'm starting to wonder as to whether I've got a hardware problem.

Latest attempt has also failed - 'An error occurred while updating system extension information'.

Edit: So have now successfully installed another copy of ElCap connected via an external drive from my 2010 MBP. It booted fine. I then put the hard disk into the 2012 MBP, and...nothing. So I hooked it externally from the 2012 machine, and it booted. Put my old hard disk (that started the problems) into the external hard disk cradle, and the 2012 machine booted. So I guess it's a hardware issue. AppleCare ran out 6 months ago :(

Edit2: Just to confirm, it was the cable that was the problem. New one fixed the issue.
Further to @Allan's advice and @klanomath's help, the fix for the issue was to replace the Hard Disk cable.