Okeedoke... I experimented all over the place. I didn't see any device step over the toes of the other. I took a clean, empty 2GB microSD card, popped it into the Nintendo DSi, copied from apps onto the SD card and created DSi folders, popped the SD card into the Tab, it created its folders, popped it back into the DSi, tried playing with it... no problems. I've tried a bunch of combinations of things and neither the Nintendo DSi nor the Galaxy Tab were botching up the filesystem in any way that prevented the other from working with the same microSD card.

My guess is that the microSD card you were using might have already had a flaky filesystem... or that maybe you weren't unmounting the SD card before you removed it and consequently corrupting the filesystem (like any Linux system, you NEED to unmount filesystems on removable devices to prevent any possible corruption). If you hadn't been doing it before, you can find the option on any Android device by going to the desktop/launcher screen, hit the Menu button below the screen, tap Settings and tap SD card and device storage. There, you'll an option to "Unmount SD card" as well as to "Mount SD card" (if already unmounted) or to "Format SD card". I wish there was a way to create a shortcut to just unmount right off the desktop but I'm told that SwitchPro Widget is the way to go to do that, if you want easy access to an icon to tap to unmount right off the desktop.