We recently bought a Cisco SG500 at work as we needed a cheap switch with 4x SFP Interfaces.
In the default configuration the SG500 only offers 2 SFP Ports for networking, the other 2 are used for stacking and they don't show up in show interface status. Howsoever if you try to configure gi28 no error is thrown. Confusing as hell.
But as mentioned before I needed 4x SFP for networking so I went on a google hunt and found nothing. The manual at least suggested that it's possible but I couldn't find out how.
Therefore I'm documenting it for everyone else here:
Warning: Back up your config, this will reset your configuration!
conf t
stack standalone
exit
write mem
reload