Reading these comments I feel obliged to mention the Android app 'DriveDroid' which lets you store multiple disk images on your phone and then acts as a bootable USB disk. I've got GParted, CloneZilla, Alpine, Debian etc live disks always in my pocket. All I need is a micro-usb cable (which most modern homes seem to have laying around) to boot into various OSs and tools.
No affiliation, I just think it's a cool free app and it's proven very handy for me loads of times.
I have been using a rooted Android phone with DriveDroid which mounts images as a USB drive for a PC. It works pretty well too. (Except for Windows setup ISOs. Those does not support direct booting from a USB drive and I had to mount a blank image and use Rufus to properly write it as a bootable non-optical disk.)
This seems to be a fine alternative that I will have to try. The problem is that I don't really have large USB sticks for this anymore...
Easy2Boot is another good one. Creates USB boot disks with multiple OS ISOs on and boot into any of them. It’s no longer being updated, but still works, just used it this week.
The only caveat is you need to make sure each ISO is stored on the USB in a contiguous file, since it tricks the host machine into thinking each ISO is separate partition. It includes a .cmd script for doing this on Windows, or on Linux you can use `rsync —preallocate` to copy the ISOs continuously.
Even on traditional BIOS systems, you can do tricky things like use isohybrid to generate an iso that can be either burned to optical media, or directly dd'd to a thumb drive and booted (using syslinux).
Another approach (my personal one) is to use a combination of grub/memdisk on a thumb drive to provide a list of bootable isos.
Hypothetically, a device that allowed you to throw a bunch of isos on a flash drive, and automatically threw up a grub-like chooser would be really nice. Yes, I can write a disk to a USB stick, but it starts to be a little clunky since you need a partition for each iso.
Ideally I want all of my install disks on one drive, since I never know if I need Ubuntu, Windows 7, Windows XP, 64 bit, 32 bit, etc. I'm tired of juggling physical media and isos.
While this works, I find this method a bit tedious to use, at least compared to Ventoy [0].
Ventoy allows you to create a bootable USB containing any number of ISO files just by writing to a partition. It even supports Windows, and has some cool features like overriding Secure Boot on some machines.
I recently discovered this tool https://www.ventoy.net/en/index.html and really like it. Essentially you just copy your iso files to the usb, and then on boot it gives you a menu to choose between them.
Speaking of booting Linux from places, what I would like to be able to do is carry a Linux image around with me on my (Android) smartphone, plug the phone into a USB port on a laptop and boot the Linux image from there on the laptop. Does such a thing exist?
You can just use Syslinux with the memdisk module (if you want to boot full ISOs stored on the thumbdrive), or you can extract the ISOs into directory trees and configure Syslinux menu options to load the contents equivalently to the ISO's original bootloader (as long as they're not using hardcoded paths or volume labels to find the filesystems to mount).
Basically it presents ISOs over the USB connection as if they were storage devices, allowing them to act as boot media for a connected PC.
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