How risky is installing Windows alongside Linux on separate NVMe drives?

Installing Windows after Linux is generally manageable, but don’t rely on the installer to respect the disk layout perfectly. If you cannot remove the other NVMe drives, double-check every disk and partition shown during setup, create a separate EFI partition on the Windows drive if appropriate, and keep recovery media plus backups available.

The ext4 data itself is unlikely to be touched because Windows doesn’t normally understand or mount ext4. The bigger concern is the EFI system partition and bootloader: Windows may place its boot files on an existing EFI partition instead of the drive you selected. Make a reliable backup first, and be prepared to restore or recreate the Linux boot entry if necessary.

The safest approach is to disconnect or disable the other drives during the Windows installation, even if that means disabling their controller or storage bus in firmware. Windows can sometimes use an EFI partition on a different disk despite being installed elsewhere. Once installation is complete, reconnect the Linux drives and choose the desired bootloader from the UEFI boot menu.

Turn off Windows Fast Startup after installation. It’s effectively a hybrid hibernation mode and can leave Windows filesystems in a state that causes problems when accessed from Linux. Also check the firmware boot order afterward, since Windows may become the default boot entry or alter the bootloader arrangement.