Nextcloud Box not using HDD for data directory

I bought a Nextcloud Box and put a Raspberry Pi 3 B into it - downloaded the image and set things up. Everything went fine until I attempted to upload files totally > 400 mb or so, when the box would freeze completely until power cycled. After some digging and investigating it appears that the default data directory for the nextcloud snap resides on the SD card (4 GB), of which there is ~ 400 mb of free space. I read the documentation on github for the nextcloud snap and attempted to move the data directory to /writable (what appears to be the 1 terrabyte hard drive included with the nextcloud box) and edit the conf.php file - however this resulted in files neither being able to be downloaded nor uploaded… the message displayed in a web browser was “Internal Server Error”. I have since started over from scratch - using the nextcloud box only for syncing calendar, contacts and notes, but I would really like to be able to use the terrabyte hard drive for more than just running the system. Can anyone provide insight on how to utilize the USB HDD more effectively?