Yeah, it’s a permissions issue
Also, your data directory is in /var/www which would be on the SD card and not the external USB drive, being only 29GB
Since it seems to be troubling to configure an SD card and external drive for you, have you considered just installing it directly on your external drive and boot directly from that, skipping the SD card?
Then it should work just like a normal install directly on the SD card, except it’s on the external drive instead
Note: You need to check if you have the supported EEPROM for this and change the boot order
There are articles about this in the NCP documentation category here on the forum
They’re also indexed at