rom manger backup not working

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http://forum.xda-developers.com – Hi, Today i root my tf700t i installed Rom Manager (free version android market) I want to instal a custom rom (i know how to do that ;) but before installing a new Rom i want to backup de original Rom. So i selected the option "Backup Current Rom in de Rom manager v apk that i have installed. The tf700t reboots and starts "updating system" but than it gave an error (update fails) and the system has to be restarted. But there is no backup Wat i am doeing wrong? Ore isn't it possible to backup the oem rom from asus tf700t? I surged arroud on the xda foru (HowTos)