I've setup an iSCSI Target on my Synology NAS and I've successfully connected to it on my Nagios Log Server via iscsiadm. The disk block has been formatted, it has a filesystem and I've mounted it to /mnt/iSCSI as can be seen below:

As you can see above, I've set up the correct folder permissions using the following commands:
chown -R nagios:nagios /mnt/iSCSI
chmod -R 775 /mnt/iSCSI
However, when trying to create the repository like so:

I get the following error:
The snapshot repository in directory /mnt/iSCSI could not be created. Possibly due to permissions. Make sure the nagios user can write to the location.
EDIT: I've also tried to enter e.g. /mnt/iSCSI/DATA to see if it would work, but to no avail.