I want to install Debian directly onto my USB drive. Is there an easy way to do this directly without having to reboot to run the installer?
So the lower-ish difficulty answer would be to run the iso installer in a VM with the usb stick forwarded to that VM.
Or you can learn what those fancy installers do: on debian you would use debootstrap
Here seems the whole guide on how to install debian manually with it:
https://gist.github.com/tr3buchet/6407920
Btw, this is also basically how you install Arch. As of until recently there wasn’t any installer and you had to go through each step manually (create partitions and fs, install the base system with <insert distro specific tool>, chroot, update fstab, distro specific finishing touches, voilà)
Depending on what OP actually wants to do it might be simpler to just install Linux normally in a VM. I recommend Hyper-v if you are using Windows Pro and if you are using Windows Home I recommend upgrading to Pro using MAS scripts or using a workaround to install Hyper-V on Home.
You could also use a hypervisor like virtualbox but they are type 2 hypervisors which are usually slower compared to type 1 like Hyper-v or KVM.
