[Bug 1955611] [NEW] Unable to boot Live USB on Mac

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Live USB boot reaches grub but fails some time after and fails to
completely boot on Macbook Pro 11,3 with Ubuntu 20.04.3

After selecting 'Ubuntu' or 'Ubuntu with safe graphics' the splash
screen shows, and after a while the screen is filled with lines of
`/init: line 49: can't open /dev/sdb: No medium found` with one line at
the bottom of `Unable to find a medium container a live file system.
Attempt interactive netboot from a URL?`

Booting with debug option from grub does not give keyboard access. I
have attached a photograph of the output at boot time to this ticket as
extracting logs from the system does not seem possible.

Full hardware specs of the exact model can be found here:
https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-
core-i7-2.5-15-dual-graphics-mid-2014-retina-display-specs.html

The iso file used is ubuntu-20.04.3-desktop-amd64.iso

The SHA256 of the downloaded iso file and MD5 sum of all files on the USB using the included md5um.txt have been confirmed OK.
All USB ports on the machine, and multiple USB devices have been tested.
The Live USB install boots to Ubuntu desktop when run from within a virtual machine under VMWare Player 16.

** Affects: syslinux (Ubuntu)
     Importance: Undecided
         Status: New

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Unable to boot Live USB on Mac
https://bugs.launchpad.net/bugs/1955611
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