You can only have one M-Card or two S-Cards installed in the Series 3. I just recently went through this with TW Los Angeles. I recently moved and when I moved they said that my previous cards wouldn't work because they had been upgraded. They only brought one M-Card and that worked as a single tuner, but when they brought an additional M-Card for the second tuner is caused the first tuner to fail. After some trial and error we discovered that M-Cards only work by themselves. If you attach another card, either S-Card or M-Card, the first tuner will fail. They came back on another day with two S-Cards and they both worked fine and I have dual tuner access again on my TiVo Series 3.