Jack,
yes, the experience that I describe has always been in my PMD-661.
Sandisk is a good reliable brand. Did you buy your cards on ebay? There are a lot of fake cards being sold on ebay, so I would avoid ebay for flash card purposes (newegg.com usually has reasonable prices and fast shipping).
on the other hand, if you didn't buy the cards from ebay, and the cards are legit Sandisk cards, I'm not sure what is going on.
One other suggestion is to always make sure that you format the card in the Marantz PMD-661 before using it.
hope this helps.
- Jason