I just flashed the unSU so I have removed SuperSU. Now trying to figure out how to get magisk on. I'll probably have to go back to stock img.
EDIT: I realize I never backed up my stock ROM before I added SuperSU.
Consider this a learning moment. You should always make a backup of the stock ROM (including boot.img) before doing any mods like root or custom ROM. It's the first thing I do, after flashing TWRP. For that matter, you should never do any of these mods, without knowing how to revert the changes, and go back to stock (either by making a TWRP backup, knowing where to get the stock recovery image, or preferably both). Always be prepared for the worst, and have an escape plan!
I'm looking thru the forum but I'm not finding the stock boot img. Do you happen to know where a link is for that?
You can download the stock recovery image from here: https://developer.nvidia.com/gameworksdownload#?search=tablet
Select the proper device (K1, LTE, WiFi - I think that last one is the 1st gen "pyro" Shield tablet), download the image, and the stock boot.img is contained in the zip.
Also in this pic, this is after I have flashed unSU to remove superSU and rebooted into recovery. I haven't flashed any ROMs to this device, only TWRP-3.2.3-0-shieldtablet.img and of course, SuperSU which I just uninstalled. So this pic I posted is when I'm trying to install Magisk-v18.1.zip. It keeps failing. Forget me trying to install any rom at this point, I can't even install magisk.
Flashing Magisk is usually very easy and almost foolproof. I assume you need to restore stock boot.img, as you already mentioned. Some folks have had success using unSU (to install Magisk). But restoring a stock boot.img is really the best/cleanest way.
Another Edit: I can successfully flash nethunter-flounder-lollipop-3.0.zip onto my tablet and run kali Nethunter with no problem.
Yet Another Edit: I just realized that I have Magisk.zip on my external SD Card. I'm not sure if it makes a difference if it needs to be on internal storage, but I'm going to give that a shot.
Pretty sure it doesn't make a difference. From what I've found, you can flash anything from TWRP (restore a backup, flash ROMs or root files) from either internal storage or external SD card