Manjaro is absolutely NOT safe when you install Windows, on a separate drive, next to Manjaro which is installed on its own drive. Last time I installed Windows I did what I never do, I let all disks plugged into the motherboard, I selected to install Windows on its own empty drive, and guess what, it destroyed my Manjaro EFI partition and boot option, on the OTHER disk. Yes, Windows, decided that as I had an EFI partition on my Manjaro disk, it would use it (pardon me, NUKE IT) to install its bootloader… Result, I had to reinstall GRUB in the Manjaro EFI from live USB, I unplugged everything but the Windows disk, installed Windows on its own disk so it created its own EFI partition on its own disk.
Ridiculous, you would think in such case it would warn you it would use an existing EFI partition on another disk, but no, it will simply wipe it…
So don’t be so sure how safe it is installing Windows…
My advice, remove all disk but the Windows one, format/install Windows on it, then boot see if it all works, then replug everything.