I have exactly the same problem with Transcend 8GB Class 6 card and Adata 16GB class 10 card. They work and test just fine on multiple PCs in multiple card readers, but in N900 they are bad news.

So far the only one that works reliably for me is Sandisk 16GB class 2, but it is extremely slow (as the class 2 rating would indicate).

It's really disappointing and I've spent a lot of money on SD cards.

I suspect maybe the N900 isn't providing enough voltage to the SD card and some cards are less tolerant of low-voltage situations than others. Does anyone know if it's possible to tell what voltage or change the voltage of the SD card in N900?