Windows bootable USB

I recently used a script (Integrate7 script) to slipstreamed all updates into Windows 7 pro - the resulting ISO was nearly 5gb. I wanted to burn it to an USB, but got a warning that the USB may not be bootable.
I searched your documentation and this forum and came across an article that you Don’t recommend using ‘Balena’ to burn Windows to USB, but I’m almost sure I did burn an ISO of Windows 7 pro to USB using BalenaEtcher about 18 month ago but I did not get the warning? and it installed on a laptop without problem? The ISO is an official one from Microsoft. I always thought that an ISO of an OS is always bootable.
Has something changed in the way ‘Etcher works’ over the last 2 years?
The slipstreaming is done in Windows 7, but the ISO to USB burning is done in Linux (Mint, or Manjaro)
The slipstreamed Windows 7 is intended for installing Windows 7 in ‘VirtualBox’ under Linux.

Hi there, Etcher does not support creating bootable USB media. You can try following this guide (for macOS) to create a bootable Windows USB stick from an image file.