My internal "Machintosh HD" is not detected by my MacBook Air I was trying to clean install OS X (from http://www.ibtimes.co.uk/how-clean-install-os-x-yosemite-via-bootable-usb-flash-drive-1470625).

I made the bootable disk, booted from it, and tried installing. Something happened and installation began. I went out for 20 mins and when I returned, I saw the installation welcome screen again. Proceeding on which I found that it came to installing it once again, but this time the internal Macintosh HD was not showing only the USB (Install OS X Yose..) was shown.

What to do? Where did my HDD go? any help?
Try and format the disk by using Disk Utility built into the installer (Utility > Disk Utility) then try and reinstall.

This should work since 'Macintosh HD' is only a volume and not the actual SSD. I used this method to fix the b-node tree error or whatever it is where I couldn't write to it.

This worked