Booting from USB on Pi3

[ZXSpectrum]

Hi Guys, Is there a program I can use so that I can boot from a MicroSD card on the PI but then have a HDD or USB Key plugged in that has a big image on it with the OS/Media?

I thought Raspberry Pi Maker would work but apparently that's mainly for multiple OS on one SD card.

Any help would be appreciated.

 

[ZXSpectrum]

Thanks Very much Mr Feek sir!