First, no matter what --- do not accept any type of system update as offered or proffered by the likes of Nintendo.
Second, remember your Wii can be picky about its SD cards. For softmodding, 2gb Sandisk (Wii-branded ones are made by them) are best and are only $10. While for other functions as you are using this card for, some applications will still be picky. Remember if your Wii is < 4.0 it won't accept or use SDHC cards correctly.
As another matter, to see what's "under the hood," please post a syscheck as outlined here as a reply within this thread.
If you look in Settings/Data Management, it should report how many blocks are free on your Wii.