The remote file systems have to be tested for access via command line. On CentOS 7 remember that it will be the 'apache' user that needs access in Moodle.
Testing example ... just one ...
Example: have moodledata on server A that has a mount point for filedir which resides on server B.
To test from command line, sudo as apache user and copy a file to filedir. Then remove that file.
sudo -u apache cp somefile /path/to/mountpoint/filedir/
Then
sudo -u apache rm /path/to/mountpoint/filedir/somefile
apache user has to have the ability to read and write so filedir has liberal permissions on the remote system - rwx.
If that works, then moodle code should be able to work.
'spirit of sharing', Ken