Using MKUsb you can grab a Ubuntu OS or Debian OS and install it on an external Usb/SSD drive this only works with the two OS's however I have not tried spinoffs like Devuan which is Debian without Systemd it probably will be fine but I have not tried it so I cannot say. To use MKUsb you must be on a debian or ubuntu system to install the utilty.
Pre-Guide
Now lets get into it I recommend having at least a 16 gig USB stick however i would choose something closer to 32 gig's you need to be familiar with nano or vim or some kind of text editor and you need Debian or Ubuntu on your system this guide will follow Debian so you will need to add a repository to sources.list
Now lets start.
1.) The sources.list file is in the /etc/apt folder so cd into that (cd /etc/apt) next because it is write protected you will need to use sudo or be root with sudo -i or su then use your text editor (nano sources.list) then add this line which adds the repository
deb http://ppa.launchpad.net/mkusb/ppa/ubuntu focal main # stable version, tested and reliable
Then run:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 54B8C8AC
This adds the gpg key to the server then run sudo apt update to see if it works if you get an error saying the gpg key is invalid you can download MKUsb directly here
https://phillw.net/isos/linux-tools/mkusb/
Download the mkusb-plug-plus-tools.tar.gz file or click the file name on this site. cd into your downloads directory and cd into the /mkusb-plug-plus-tools then cd into /plug-dir then run:
sudo ./installer i
this will install it on your Debian system.
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