Files on external storage and moving them

Hello Community,

I have an Nextcloudpi Setup on a RPi4 with 8GB RAM and a connected NVMe HDD with 1TB over USB3.0. The Nextcloud Image is installed on a 32GB Samsung Evo-Plus SD Card (Image: ownyourbits).

I know when i copy from my laptop or Any other device data to my nextcloud i have to manually execute:
sudo -u www-data /usr/bin/php /var/www/nextcloud/occ files:scan --all

That works great and i can also setup a script which will do that via cronjob:

Is it right that i cannot disconnect my ext drive from nextcloud and connect it to my macbook?
And how about when my nextcloudsetup is anyway damaged and i have to reinstall everything. Can i afterwards connect my ext Drive and Access all files?

Is it better to have also ncdata, nc-database, swap and zram on ext drive? Or Better to have only the files on ext. Drive??

What is the Best Backup Method to sync files to another ext. Storage? Manually create rsync script or is that integrated in pi webpanel? I sam iz already Bit did Not understand correctly