The drive should have "Ignore Ownership" checked, but I'm not sure if that can be done for a boot drive even when booting from an external drive.
Once you know that you have all your data on the new drive, then I would erase the internal drive. It helps to better separate your external drive from the internal. If you happen to accidentally save something to the internal drive, then it is much easier to notice if the internal drive is blank than if you have all the macOS files/folders on it. Plus if the PRAM is reset, it will reset the default Startup Disk where it would then try to boot from the internal drive.
I would probably even go one step further and prevent the internal drive from even being mounted so that it won't be seen an option when saving files. This requires the use of the command line to edit the "fstab" file. I just tested this on Big Sur for not auto mounting an external APFS volume.
Launch the Apple System Profiler (Option-click the Apple menu and select the first item). Click on the "Storage" item on the left pane. On the upper right pane, select the mounted internal volume. You need to highlight the long alphanumeric value found after "Volume UUID:" (it will look something like "6B55EDA6-76AB-4DA1-8725-9451848CAF9A") and copy it.
Launch the Terminal app and type the following command to launch the editor so you can modify the "fstab" file properly:
EDITOR=nano sudo vifs
Make sure to note the upper case & lower case characters. You will be prompted for your admin password. Nothing will appear on the screen as you type the password, so just press the "Return" key to submit the password.
Once the editor is launched in the Terminal window, use the down arrow key on the keyboard to move the pointer to the end of the document (the trackpad pointer has no bearing on the cursor for the editor). Begin typing "UUID=". Now paste the UUID value you copied from the Apple System Profiler by using Command + V (or click the "Edit" menu for the Terminal app and select "Paste").
Now press the Tab key and type "none", then press the Tab key again and type "apfs" if it is an APFS volume. Press the Tab key once more and type "noauto"
Assuming the internal volume is an APFS volume with a "Volume UUID" of "6B55EDA6-76AB-4DA1-8725-9451848CAF9A", then the text within the editor should look like this (you will have a different value for UUID):
#
# Warning - this file should only be modified with vifs(8)
#
# Failure to do so is unsupported and may be destructive.
#
UUID=6B55EDA6-76AB-4DA1-8725-9451848CAF9A none apfs noauto
To save the updated contents you need to press Control + X.
Note: The bottom two lines of the editor window will list several common key commands used with the editor. The "^" part signifies the Control key.
You will see a prompt at the bottom of the editor window within the Terminal app window say:
Save modified buffer (ANSWERING "No" WILL DESTROY CHANGES) ?
Y Yes
N No ^C Cancel
If you decide to abort the new entry, then just press "N", otherwise if the line looks correct, then just press "Y" (you can respond with a lower case "Y" or "N"). It will then ask you another question:
File Name to Write: /etc/fstab
Just press the "Return" key to accept this file name & location.
If you decide you don't want to prevent the internal drive from mounting when you boot the computer (that is you want to restore default behavior), then all you need to do is relaunch the text editor from the Terminal app, use the down arrow key to move the cursor to the beginning of the line you added and type a "#" character so the line starts with the "#" character like the other lines, since this make the line a comment line which is ignored by the system. Just make sure to exit & save like before.
Keep in mind if you ever erase the drive again, then the resulting volume will have a different UUID value so if you still wanted to prevent it from auto mounting, then you would need to edit the "fstab" file again by providing the updated UUID value.
If you prevent the internal drive from auto mounting at boot....you can always mount it manually later on by launching Disk Utility....select the grayed out volume on the left pane of Disk Utility, then click the "Mount" button on the right pane. You can eject the drive as usual using the Finder.
If you instead erased the internal drive as MacOS Extended (Journaled), then you would follow all of the above steps, but you would replace the "apfs" part with "hfs" instead.
Also, this is the only safe way to edit the "fstab" file.
