Boot from USB flash drive on iMac 2017

I will offer some thoughts to consider.

• Flash drives are not really that reliable and in my opinion, not suitable for use as a startup drive.
• Using a Flash drive may be somewhat slow.
• I suspect that something is wrong with either your Flash drive or the OS file which is on it.
• You say"I know the drive is working fine, I can access it on finder." If it were working fine it would work for you. It does not.
• I suggest reformatting the Flash drive and reinstalling the OS. That may resolve your problem.
• I use external SSDs with operating systems on them and they are reliable and quite fast.

I hope something here is helpful.

This may help - How to Boot a Linux Live USB Drive on Your Mac. Just remember this was written in 2017, when Sierra was the current version of macOS. Things may have changed since then, with the significant architectural changes to macOS especially in Catalina and later versions.

What Linux distribution? Some Linux distributions may not be easily bootable on a Mac due to how some distributions may name their boot loader. Many non-Apple PCs have issues but can be worked around due to having a BIOS interface where modifications can be made such as searching for the Linux bootloader, but Macs don't have any options when this happens. Linux Mint, Debian, and Ubuntu flavors are known to work with Macs (the installers should boot, but sometimes once Linux is installed onto the Mac you may not see a boot option without modifying the bootloader file & folder names). I don't know which other distributions may work since these are the only ones I have ever tried on a Mac (or know someone who has used them on a Mac).

In fact, I have encountered issues where the Linux installer for a particular distribution may work fine, but once Linux has been installed, the Mac (even some regular PCs) won't see the Linux installation as a boot option, since the bootloader name has been customized in order to properly identifier which bootloader belongs to which OS, but UEFI firmware on many PCs and on Macs, don't always recognize these bootloaders and will only recognize the default simple name "boot" and "bootx64.efi" instead of "ubuntu" and "grubx64.efi". UEFI firmware is only made to work with Windows or with Macs to work with macOS...both vendors only care about the product they sell not caring that someone may want to have another OS.

Etcher (Mac/Windows/Linux) is a great option for creating a bootable Linux USB stick as it just transfers the raw image directly to the destination drive. Some of the other options mentioned in the article linked by @MartinR may actually modify the boot disk behavior...may be Ok, other times maybe not.

I was able to boot Debian unstable SID on a 2020 iMac (there were issues, but it booted...this was an external drive I had created for a non-Apple PC). The 2020 iMac has the T2 security chip which has a lot of compatibility issues with Linux at the moment, but a 2017 is just a standard system so should work much better with Linux.

booting from USB flash drive

Is this a macOS bootable installer flash drive?

If so, it may have become corrupted and not bootable. You may need to re-create it.