MacBook Pro won't start. Question mark folder shown.

Hi,

 

My MBP won't start after a full battery drain when playing music on iTunes. A blinking question mark folder appears.

 

- Running Mac OS Mavericks at time of failure.

- Currently Mac running OS Sierra from USB drive.

- Mac won't recognise HDD connected on SATA interface.

- Outside USB diagnostic says mentioned HDD works fine (Mac Original Seagate 500 GB).

- NVRAM and SMC have been already reset.

- Internal HDD feels spinning (HDD Cable seems fine).

- Other SATA interface (Optical Drive) working fine.

 

 

For any suggestions, thank you.

 

- David

MacBook Pro (13-inch Early 2011), OS X Mavericks (10.9), Model: A1278