I am using Maemo 5, Version 10.2010.19-1 on N900.

When I am using tar to pack files with
tar -cvzf /media/mmc1/backup.tar.gz /
or
tar -cvf /media/mmc1/backup.tar /
onto the SD card as root user I am getting
"tar: short read"
and the tar file is garbage.
:-(

What can I do?
(I am used to Linux so if there is an alternative I can try please tell me, but normally tar should work on a Linux system!)