Comment 27 for bug 500822

Ubuntu 18.10 install from USB on a Dell Optiplex 7040 Micro. I was getting the same error (mostly), but here's what fixed it for me...

I had a USB flash drive with the installation on it (followed directions from https://tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-windows#0). This flash drive and a USB keyboard were plugged into the front USB ports. Would try both "Install Ubuntu" and "Try before installing" and would get the message: (initramfs) unable to find a live medium containing a live file system.

From the GRUB menu, I edited the parameters for the installation, removing the "quiet splash ---" from the line to install, then booted to that option. Got the repeated message "/init: line 7: can't open /dev/sdb: No medium found" repeatedly.

After fumbling around a bit, I guessed that it was some weird APCI setting that I couldn't change, so on a whim, while it was repeating that 'No medium found message', I swapped the ports that the flash drive and keyboard were plugged into, and it started booting up...

[MAY BE TOTALLY WRONG]: I think that somewhere along the way, the power to the port where the flash drive is plugged in has its power turned off or something. Port for the KB is on. Maybe the action of plugging the flash drive into that specific port fixed it, or maybe the action of plugging it back in to any port caused it to re-mount, I'm not sure. Just wanted to share what did it for me...

TL;DR - When the installation starts firing that "/init: line 7: can't open /dev/sr0: No medium found" repeatedly, swap the flash drive and keyboard USB ports.