I too have had some issues with my sd cards. the following is what I posted into the "metecrawler" posts but I think that it has some bearing here as well. I have yet to destroy any cards but I think that I may have come close. here is my post:

"I am a newbe, but I have been following these and all other posts that have to do with battery life and sd card failures. I have had my n800 for about 5 months now. I have been using 2 [two] 2gig sansdisk sd cards continuously filling up, moving around, and swapping out files including movies, pictures, pdfs, and a lot of use with Mam Mapper. Until the os update that is. After the update I wuikly noticed that battery life tanked. Like the above posts..very quickly. I started to play around with the sd cards...ie: empty, full, partial etc... it did not matter. I found too that I began to have a great deal of apps that would hang forcing reboot after reboot. I have finally just put back in the nokia micro sd that the n800 came with. I have no card in the external slot. Battery life is back to what it was when new. As soon as I put another card in the external or internal slot...I immediatly lose hald the battery and apps [does not matter which ones] begin to hang. none of this happened before the os update. I have not and will not try a larger size sd card until this is fully resolved. I would appreciate any input as well. I am not a linux guy or programmer of any type. Just a consumer who thinks the n800 is an incredible little piece of tech.."