If you have run into this issue please upgrade to 4.1.10: http://www.readynas.com/RAIDiator_4_1_10_Notes
You can then use the Advanced Options tab (go to Shares > Share Listing > Sharename then click on e.g. CIFS and you'll see the tab) to reset the ownership to what you want. You can use the Set ownership and permission for existing... option. Don't forget to click Apply to make your changes
It appears that I am unable to change the "Share folder group" while running 4.1.9 [t6]. I previously had it set to "users" which is the group my users are in on my ReadyNAS Duo. However when I upgraded to 4.1.9 [t6] this changed to "nogroup" and despite numerous attempts to change it back to "users" it fails, each time when I click "Apply" the webpage submits and a second or two later when it refreshes it reverts to "nogroup". The result of this is that unless I set "share folder everyone rights" to "Read/write" my users do not have access to create files and folders in the root of the share, sub-folders on which they previously had permissions are OK but the root is effectively read-only. If I downgrade to 4.1.8 I can change the "Share folder group" and regain write access to the root, so it appears to be an issue with 4.1.9 [t6] unfortunately.
If I can provide any further logs or information, or run any tests, please do not hesitate to ask.
Kind regards, Alec
