Error creating/copying data to externally mounted storage

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Error creating/copying data to externally mounted storage

I’ve setup nextcloud using ubuntu appliance on RPI 3 and it is working very fine.
Recently I mounted an external HDD and added it as external storage to nextcloud and it detected and shows added as well, but when I try to copy data to it I get the following error:
Nextcloud error Error while copying file to target location (copied bytes: -1, expected filesize: 272931 )

I followed this article- Raspberry Pi 3 Nextcloud Box running on Ubuntu Core | Flexion.Org
Where am I going wrong?