installing linux mint on external ssd drive on MacBook Pro 2017

Linux support for the USB-C Macs is spotty at best. A lot of things may not work (especially WiFi). Linux Mint does install and work fine on the older 2007 - 2015 Macs and it is always safer to install to an external drive to minimize risk to the macOS installation. Running Linux in a Virtual Machine (VM) is the safest option as long as the shared resources and lack of GPU hardware acceleration is acceptable (for most things this won't be a problem). I did install Ubuntu to an external drive which did boot a USB-C Mac, but I only booted it one time to see if that version of Ubuntu would work at the time since I like to use Linux to test computers I'm working on. Linux Mint is based on Ubuntu (or Debian if using the LMD edition). I never recommend installing another OS to the same boot drive (especially the main internal boot drive because modifying partitions is always risky and could lead to each OS being unbootable and possibly loss of data (or at least access to data).

Here are a couple of sites with information about the status of Linux support on the 2016+ USB-C Macs:

https://github.com/Dunedan/mbp-2016-linux

https://t2linux.org/

I managed to get LM up and running a couple of years ago on my 2015 iMac 21.5" Retina via VMWare Fusion.

I installed VMF on the Mac's internal drive and had the actual LM VM file on an external SSD. I no longer use Linux, but that same SSD arrangement runs Windows 10 beautifully.

As far as I remember LM ran OK. Kept telling me I had the wrong display drivers or something and it was running in virtual mode or some such and would be very slow. All I could do was stop the notification; speed was fine I recall.

Most Linux distributions contain all the available open source and even a few closed sourced binary drivers in their software repositories. If some Apple hardware on the laptop is not functioning, then use those articles I provided as a guide if a specific driver needs to be installed to operate some feature on this Mac. Usually when working with drivers you need to identify the exact hardware identifier of the hardware needing a driver which is usually done through the command line to get the specific information which can then be used to locate the driver in the Linux distribution's software repositories or to get more information/assistance when searching online.

For some hardware peripherals the product manufacturer may provide Linux drivers (usually binary only) from their website, but the majority of time the Linux distribution's software repositories will contain at the very least a basic open source driver. Which is better to use a lot of times depends on the situation. I personally try to use the distribution's drivers since they usually don't cause problems when updating the Linux kernel which many times can cause a binary only driver to break. This is something you will need to research or ask on Linux dedicated forums if the need ever arises where you need to install a driver to make the hardware work.

If you are booting macOS and Linux on bare metal, then they don't interfere with one another since only one OS is running at any one time (one of the downsides of dual booting). When running one OS in a VM, then yes, the two operating systems will be competing for limited resources such as memory & CPU time, plus GPU hardware acceleration is usually not available in a VM which may affect the performance of both operating systems.

I provided the T2 link just for reference as its information may be more up to date than the other link (some items may overlap a bit with some older models like perhaps WiFi), plus just to be safe since a lot of people post the wrong model of their Macs. There is not much information on Linux support for USB-C Macs, and those two links were the only ones available last time I searched (there was another one, but it was outdated & superseded by the Dundean link.

As @Grant mentions, for Linux issues you will usually be better served on a Linux forum or a more general technical forum, but keep in mind on those forums there probably won't be many users who are familiar with Apple hardware. Most times a user will need to take general Linux information & procedures and apply it to their own specific case and figure it out (although with enough details users on a Linux forum will be glad to try assisting). This is another reason why using Linux in a VM is better than on bare metal (at least on newer Apple hardware).

Wi-Fi support for Virtual Machines and alternate booting is extremely complex, because Wi-Fi uses Hardware IP address filters on all incoming messages. Expanding an existing Wi-Fi connection to get another machine connected is really hard to do.

It you can instead use Ethernet as your main computer connection, "bridging" onto that Ethernet is a cinch, and works well. And it's generally faster as well.

I was actually referring to a bare metal install regarding the WiFi compatibility. I guess I interwove the two methods in my post. I haven't personally installed an OS in a VM for a long time, but thought it would interface with the macOS connection.

I should have mentioned in my other post that it is possible to install the VM software on the main boot drive, but have the actual VM OS container located on a fast external drive. I was a bit distracted by personal stuff here today.

Most Readers here are working on keeping MacOS working on their Macintosh Hardware.

You would probably be better served by looking for a Linux forum, and find a topic about installing linux variant you want on Mac Hardware.