Here's an idea: why don't you unplug the hard drive from the motherboard before starting the PC? You are talking about a desktop one, I guess, is that correct? If so, and if the warranty has expired, open the lid and unplug the SATA cable of the HDD. Then, start the comptuer and try to boot from the USB. If you have more than one USB port or rear one, try all of them.
After that, plug the SATA cable of the HDD again /or in another SATA port of you have more/ and try the installation process one again.
Let me know if that helped.