It depends on the version. v1 players can support up to 2GB. v2 players can support up to 8GB.
You
can find out what version you have by going into Settings -> System
and look at the version. If it starts with a 01 then it's a v1, if it
starts with an 03 then it's a v2.
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You
can use a card up to 8GB if it's a v1 player by installing Rockbox on
it. It's a 3rd party firmware for some mp3 players - although you
should make sure to read any instructions for installing it, as well as
read the manual.
->
Rockbox.orgI will stress this again: make sure you know what you are doing by reading installation instructions and the manual.
In
order to install Rockbox on your player it needs to be in MSC mode
(Settings -> USB -> MSC). If your player doesn't have the USB
option in the Settings menu then you will need to downgrade your
firmware. There's info on this in the Sandisk forums about how to do
this.