Buffalo TS-XL User Manual page 44

Once you check Access Restrictions, you must specifically add
access rights for any user or group to access the share. To add
rights, choose either the Local Users or Local Groups tab. If you
are using AD, then you will have additional tabs for Domain
Users and Domain Groups. If you're using SMB, then you'll have
an additional tab for External Users. Click on any of these tabs
and then click Add.
Put a check next to any users or groups that you want to give
access to the share. Click Add when done.