Restore data and repair an un-bootable Macintosh HD startup disk

rahult@apple wrote:

2. restart but press command-R option to enter recovery mode. With this, I tried to reinstall MacOS but it does not show Macintosh HD as installation disk. The only disk that shows up is the recovery disk, which shows up as encrypted.

Sounds like you ran the installer from within macOS. From Recovery Mode launch Disk Utility. You may first need to unlock the drive/volume using Disk Utility before the macOS installer will recognize the volume as a valid destination. You should also try running Disk Utility First Aid on the drive to check for problems.

So my question is: how do I repair my Macintosh HD startup disk without loosing my existing data. For now, I am currently installing Mac OS on an external hard drive and hoping that once I am able to boot from external HD, I should be able to access my existing data and back it up on the external drive.

i would appreciate any help on my question. Thanks.

This sounds like the best plan. Once you have backed up your data, then I would suggest erasing the physical drive as GUID partition and APFS (top option) before reinstalling macOS. Within Disk Utility click on "View" and select "Show all devices". Your internal physical drive should now appear in the left pane of Disk Utility. Make sure to select the physical drive to erase and not just the "Macintosh HD" volume.

You may want to create a bootable macOS USB installer using these instructions:

https://support.apple.com/en-us/HT201372

If you have trouble creating a bootable High Sierra USB installer, then let us know as I have an alternative method if these instructions do not work and Recovery Mode (Command + R) do not work.

What is the exact model of your computer? You can get this information by clicking on the Apple menu and selecting "About this Mac" or by entering your serial number here.

Please startup backing up your system regularly. You never know what may happen.