HDD failure or something else?

Hi, I have  a XPS One 2710.

it, 32GB SSD, 3.5" 2TB 7200RPM SARA HDD, NVIDIA GT640M video, 2 x 8GB DDR3 SO-DIMM @ 1600mnx, Win 8.1.  All the drivers, BIOS etc are up to date.  About 2 years old.

Recently, it's taken to random shut downs.  2 variants:

1. Will crash when in sleep mode, and I arrive to a dos prompt message along the lines of "can't boot from the device...  f2 to enter setup (i.e. BIOS) or attempt to reboot".   It wont  boot on attempt... only way id to power off.  and then it will boot into win no problems

2. Whilst in use, becomes unresponsive (intermittent), then a general freeze.  I can also hear a very faint beep from the back of the machine.  Then it blue screens.  Then it will restart / boot form here.

I have noticed on entering BIOS after it crashed that the HDD isn't recognized.  So, I can see why it can't find the boot device.  If I change boot device order... eventually the mSata HDD registers in the Devices section.  This seems odd?

I've run all the Dell diagnostics (all clear) , re-installed windows etc... and still happening. 

Any ideas?

Thanks