I finally found time to re-check everything and re-uploaded the file.
The proper size of update.img is 314.572.800 Bytes.
And - more important - the file system of update.img somehow was corrupted, the files responsible for flashing the reader were not corrupted at last, but maybe this was the cause for the sd recovery not working... I recreated it and ran fsck on it, should be ok now.
Last edited by uboot; 06-12-2012 at 05:08 PM.