When trying to boot from usb, usb does not show up?

I've seen posts on these forums with people using Kali Linux on a Mac, of course there could always be a bug with the most recent version or the current download is corrupt...verify the hash to make sure the .iso is good.

The USB stick is the weakest link.

FYI, I never recommend dual booting using only the internal drive because people almost always find out one or both operating systems doesn't have enough room, or they no longer want the second OS. Fixing this requires modifying partitions again which is always risky and many times will involve needing to start over completely.

If you want to use Linux on bare metal, then install Linux to an external USB3 SSD which should minimize the risk to the main internal boot drive. Otherwise installing Linux into a VM is a good choice depending on how it will be used and the system resources of your Mac and it minimizes the risk to the main OS.

Just make sure to have a good backup in case something goes wrong.

As for Knoppix, you may want to try v8.6.1 to be safe if it is still available. I've only tried using the v9.x series a few times and not sure if the issue is with Knoppix or the hardware I was using at the time. Unfortunately I don't have much free time to experiment these days to confirm.

What is supposed to be on the bootable USB stick?

How did you create the bootable USB stick?

Try using the other USB port.

Disconnect all other external devices in case one of them is causing a problem.

Connect the USB stick directly to the laptop.

Try using another USB stick especially a different brand since the quality of USB sticks is extremely poor, plus Macs can be very picky about the drives used for booting....assuming you created the bootable USB stick correctly.

If you are trying to create a bootable macOS USB installer, then here is an Apple article with the proper instructions:

Create a bootable installer for macOS - Apple Support

What Linux distribution are you trying to use? Not all of them are compatible with a Mac.

Etcher should make a bootable Linux USB stick without any issues assuming the Linux .iso image is made to boot a UEFI computer since Etcher just transfers the raw contents of the .iso file directly to the USB stick (the same as using the command line utility "dd").

Try a PRAM Reset making sure to hold it for at least three chimes. Sometimes an NVRAM issue can cause weird problems.

I believe Ubuntu (and its various flavors), Linux Mint, and Debian all have USB installers which will boot a Mac (at least with the USB installer....Debian may not boot after installation without a bit of tweaking the bootloader file & folder on the hidden ESP partition). Theoretically all other distributions should also work, but it all depends on how the maintainer of the USB installer image created it.

You are using the entire USB stick when using Etcher (I know this is the default behavior)?

Hi,

I just wonder if none bootable data USB is your MacBook Pro detected or not.

HWTech once recommended [self-configuring] Knoppix "live" booted from a USB stick. I am not a unix guy, but was able to 'make it go' easily.