Macbook Alu system disk won’t work with InstaDMG.

Hi all. I'm trying to do a special workflow for the news Macbooks but get stuck right away. I did a dmg of Disc 1 via Disk Utility, dropped it in the baseOS folder, did a copy of my base updates with some slight changes. Fired upp instadmg (b4) and instantly i get this error: [i]08:02:21 ######InstaDMG build initiated###### 08:02:21 ######Mounting Mac OS X installer image###### Mounting a support disk from ./InstallerFiles/BaseOS/./InstallerFiles/BaseOS/mabook_alu.dmg ERROR: the main install disk was not sucessfully mounted![/i] I made a fresh new dmg of Disc 1 just to make sure that its not the dmg. The dmg mounts on my desktop just before i get this error. Any ideas? Many thanks. p