Please check that the data directory contains a file ".ocdata" in its root

About

This message is generated by ownCloud, ostensibly when it does not find the .ocdata file in its data folder – but the error can occur even when the file does actually exist if ownCloud can't access it.

Solutions

• What worked for me was changing the ownership of just one folder in the data directory path.
  • The path in question is /mnt/storage/www-data/owncloud, where the "storage" folder (actually a mountpoint) is an alias for a large secondary drive. For some reason, that "storage" folder (mountpoint) was owned by my normal-user username and permissions were set to 700. There may be some way to set the permissions explicitly in fstab, but changing the mountpoint's permissions to 777 after mounting seemed to be "sticky", i.e. umounting and remounting it preserved those permissions (even though in the unmounted state, the "storage" folder had 775 permissions instead of 777).
  • Short version: Tentatively, every folder in the path must provide at least read permission.

Attempts

Things which didn't work:

• chown www-data:www-data .ocdata
• chmod 666 .ocdata
• Creating the file at every parent folder in the path.