Just to add..

The card I'm suffering with is in fact a Sandisk 32gig card..

The card works in my (symbian) Nokia N97, 5800
The card works in my Android Nexus one
The card works in all my card readers (Windows, Mac and Linux.. various hardware)

The card works in a Fuji Camera (via microsd to sd adapter)

The card does not work correctly in my N900 or a friend's N900

I believe the card works correctly when mounted read only, it seems the errors occur after the first write action (which itself occurs when the card is mounted, before an filesystem copy/move actions occur).. I'll double check that later..

dd if=/dev/zero of=/dev/mmcblk1 bs=1024k
dd of=/dev/null if=/dev/mmcblk1 bs=1024k
both ran without problems provided the card was not mounted first.

I've not tried using the card in mass storage mode via the N900.. I suspect errors might occur though as the problem appears to be the mmc controller

The fact the card a) does not work in two N900 systems and b) is a not officially supported 32gig size may suggest I'm pushing my luck or that the fault is on the card..

The fact the card has worked in about 8 or so other devices (phones, cameras, other) just irritates me more.. I swear the Nokia tablets have had crummy mmc interface equipment / drivers..

my investigation continues (and I may raise an RMA for the card)

All my other SD cards (2,8 and 16, various classes, all Sandisk) appear to work without problems in the N900

Since OP reports problems with a 16 gig card, I get the feeling some cards under some circumstances won't work.. and the N900, as previous tablets were, is just too fussy