First off - feel free to laugh or shout about this, but please do it in private.
Years ago when I was first using Linux and windows together, we used used a floppy disk to hold a boot manager pointing to a secondary HD with a Linux distro. I believe it was using grub, If I had a working 5-1/4" drive I would check one of these old disks.
This allowed me have a Linux OS tucked away on a partition or secondary drive and still keep the native OS boot manger untouched. So if the floppy disk was not inserted, the original OS would boot. I often used this to rebuild training rooms systems from a snapshot stored within a Slackware partition.
Is there a modern equivalent of this tactic with out having to build a custom ISO or installing full Linux distro to a USB? I just need the boot manager to be on a USB memory stick or CD to point to an installation on a secondary drive. Currently there is no need to even boot to the existing native OS on the system when the USB memory stick or CD is in.
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partition on a secondary drive and you can install the bootloader in the USB instead of your harddisk.