chan's fat read problem at 4096 bytes

Hi,

I'm experimenting with the chan's fat system on a sam7X.  I am reading a 
rather big file in segments in a buffer of 512 bytes, when the file 
pointer arrives at 4096 bytes the read stops and the clust long in the 
f_read function retrieved from the get_cluster function is 0xFFFFFFFF. 
anyone seen this before?

thanks,

Peter