Here's a tutorial that I recorded that goes through how to install an entire Linux operating system on an external hard drive using a free OS virtualization program called Virtualbox. This video's kinda rough, as I had to stitch together both real life video footage (from my phone, haha) as well as desktop recordings. The actual end product runs a lot smoother than my phone makes it seem, of course.
PLEASE MAKE A NOTE: I'm using a completely and utterly empty external drive here, so I can't really comment on what would happen if you had data on it already and wanted to resize the existing partition (I'd assume nothing, if you repartitioned it fine without it failing or anything). If that's your case, look up a quickie little tutorial on repartitioning your drive without losing your data. Note, also, that I'm a complete noob at this, so I can't comment on how feasible it is to use an external drive for an entire OS for an extended period of time (i.e. all day, every day, for years). It's also worth mentioning again that installing an OS via Virtualbox (which is a free OS virtualization program) will probably be substantially slower than installing it "on real hardware", though in my example, I believe it took around 35 - 40 minutes, which isn't too long. This, of course, would vary based on your computer specs.
I forgot to mention that you should choose Something Else for the partitioning process if you already have data on the drive so that you don't end up erasing the data. Of course, you'll have to do it yourself as I did in the video above.
I wanted to record a tutorial to talk about this since I think it's a cool hack that could prove really useful for people (like me) who want to try out Linux without going through the live CD / USB creation steps, or who want to try out and install multiple OSes in succession without having to worry about accidentally erasing their data.
Important point book-marks:
1:28 - Pros of the process
4:58 - Cons of the process (I forgot to mention that installing via Virtaulbox is slower, but whatevs)
10:54 - Basic Windows partitioning of unallocated space on external hard drive
16:30 - Creation of "Ghost" Virtualbox virtual OS to install ISO onto hard drive
17:56 - Selection of Linux OS ISO / External HDD
22:18 - Beginning of actual Linux OS installation
23:38 - Beginning of Linux Partitioning
29:04 - Results
32:23 - In case of "Non system disk; press any key to reboot"
35:04 - The most amazing thing you will ever witness
--- LINKS ---
Here's that HowToGeek page that mentions how to partition your drive:
Here's a link to Zorin OS's homepage:
Here's the Virtualbox (the free cross-platform open-source OS virtualization software) homepage:
Here's my Patreon page, as well, in case you want to tip me for my creation of tutorial videos like these:
Thanks a lot!