This is a feature request based on this comment in a related issue.

This Rufus issue describes the feature, in which when an issue is detected, the user is prompted to fix the boot support by retrieving ldlinux.sys and ldlinux.bss files to include in the image being flashed. For affected ISOs that lack those files, the USB media does not appear to be bootable(eg Clonezilla via Etcher fails, via Rufus is successful).

Clonezilla Live is a useful backup tool to have on a USB stick, it's cross-platform, but I am only aware of Rufus(which is only available on Windows) as a tool that can support making the ISO work on a USB device which is much nicer than the Clonezilla documentation for a zip file alternative.

It would be fantastic if Etcher could support this, I believe I've seen this on a few other ISOs in past years, so while the issue may be in how Clonezilla produces it's ISO images and it may be their responsibility to address, it would be great if Etcher could support this fallback for compatibility like Rufus does.