Problem remounting /system as writable

[Guest pajn]

I can't remount /system as writable.

adb remount doesn't help, I don't get any error but /system is still write protected.

Same with mount -o remount,rw -t yaffs2 /dev/block/mtdblock4 /system

The device isn't rooted but I do have root access from adb (line begins with #)