Hello Matthias_Schmidt,
hope this finds you well.
for my experience, i had to setup the external HDD as FAT32 and then configure the automount to an specific folder (/media/nextcloud or something similar) and then gave permissions as www-data.
one time i plugged the external HDD to a different port and it wasnt even recognized by the system, so i had to plug it back again to the original USB port i configured originally and voila!.
i hope this helps a bit…
Regards!!