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Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 29.0.5): 29.0.4
Operating system and version (eg, Ubuntu 24.04): 24.04
Apache or nginx version (eg, Apache 2.4.25): replace me
PHP version (eg, 8.3): 8.2.20
I installed my nextcloud using on my Ubuntu machine.
sudo snap install next cloud
Everything works fine except the external storage drive. In my case I have a 1TB usb harddisk that is connected to the computer where nextcloud is installed. I have mounted the harddisk at:
/mnt/usbdrive
I also assured that the owner of the folders from mnt and its subfolders are www-data:www-data using this command:
sudo chown www-data:www-data -R /mnt
sudo chmod 775 -R /mnt
I checked that the folders from mnt and its subfolders all are owned by www-data:www-data and the permissions are correct.
From the Nextcloud’s web app, I setup the external storage app, and setup the folder location at /mnt/usbdrive. I see the green tick indicating that the setup is successful. However, when I from Files entered my usbdrive, I don’t see any files.
Upon checking the log I see
[PHP] Fejl: opendir(/mnt/usbdrive/): Failed to open directory: Permission denied at /snap/nextcloud/43563/htdocs/lib/private/Files/Storage/Local.php#164
PROPFIND /remote.php/dav/files/JianWu/newUSB/
from 94.147.65.45 by JianWu at 25. aug. 2024, 08.33.10