@ MSMG: Thanks for your phantastic Toolkit, the script and your guide. @ all: It was no problem at all to slipstream the 4+2 Win8.1 x64 Spring Update hotfixes into the original Win8.1 Pro x64 de-de Retail RTM ISO file. Everything worked without any issue. Only problem for me: The size of the ISO file I got at the end was 4.884 MB, that means much too big for a FAT32 formatted USB Flash drive, which I need to get the OS install in EFI mode. Solution: I simply converted the install.wim (size here: 4.242 MB) to an install.esd file (new size: 2.729 MB) by using the following command: Code: dism /Export-Image /SourceImageFile:"D:\install.wim" /SourceIndex:1 /DestinationImageFile:"D:\install.esd" /Compress:recovery Then i have done the following: 1. Opened the ISO file, which had been created by MSMG's Toolkit, with UltraISO. 2. Copied the freshly created install.esd into the Sources folder. 3. Deleted the install.wim. 4. Saved the modified ISO file (new size: 3.4126 MB). 5. Created an EFI mode bootable OS image by using the tool Rufus. The installation of the OS by booting off the USB flash drive and the activation with my Win8.1 Pro Retail CD Key worked flawlessly.