- Always try to upgrade to the newest IOS when using gigastack
- Connecting both stacking cables will have switches revert to half duplex.
So my advise would be : connect only 1 cable from switch to switch, upgrade both ios, reboot. Check if you see a green light on both gigastacks (if you connect both cables, you should get 3 green lights 1 red for spanning tree block)
btw. not sure what you mean with "changed to master".