16gb CF D3 and 32gb SD D600. always use 2 and NEVER used in overflow. most use their cameras for their hobbies and use overflow, but as a pro, 2 slot cameras are mandatory and always in backup mode. when I was looking for a 2nd FF camera, I did look at the D700, but realized the 1 slot would be the death of me if a card went rogue. pay more and go with 2 slots. it only takes 1 time to regret that decision.
summer 2013, we shot a wedding and the videographer shot wiht a canon DX (I think 60d) 1 slot. the card decided to go on vacation and the bride/groom first meet file was corrupt. I dont know the end of it but its a chance to shoot with 1 card. your reputation is on the line.
an older pair of 8gb extreme III behaved weirdly when I transferred a bris I shot to my PC. while it was transferring, it was hanging on some pics for a very long time. started the rma process today and can no longer rely o them. when it behaves weirdly, thats it. it the beginning of the end. that was the sign its defective and must be replaced. this time I got away, next time itll be a headache and recovery software. third time, corrupt files and no recover.
same with HDD. the second you see it behaving weirdly, dont doubt. get a new HDD, transfer all your pics and have it replaced if under warranty. I want to get into the habit of replacing my memory cards once a year. sell it for 50% and get new ones. I can also just say the cards are defective and do an RMA but that process is just tiring.