How to fix the booting to Aptio Setup Utility BIOS issue with ASUS X555 laptops.
This problem is caused by the HDD board no longer having a reliable connection to the mainboard. Replacing the HDD board often resolves this, however in some instances it is necessary to instead swap the HDD into the optical bay in a caddy.
The various X555 models have different HDD boards, so it is important to make sure the correct part is purchased - I sourced the replacement from ASUSParts.eu which has a good part finder - the HDD board is listed under "Sub-boards": http://www.asusparts.eu/en/partfinder/Asus