Can not boot other OS once volumio is installed

I installed volumio on a 2010 mac mini. It worked to some extent, but I want my own solution instead, so I am now trying to install another linux distro.
When I try to boot from the freshly created USB drive, it still falls back to boots volumio from the hard drive. I pick UEFI boot, and it still boots from hard drive. I don’t know how you volumio guys managed to do this, but I would love to get back my computer.

Probably your Usb drive can’t boot because something is wrong when you flash it
Even if volumio is installed on your hd, there’s no trick in it to boot from usb is you set your computer to do it…

Uh, ok, it was two bad boot images, I did not think this happens with two independent sources in sequence. Thanks for the reply, it helped.