Hi @cantfind ,
Welcome to the forum!
Maybe, the problem is there and you did not change the fat32 partition to /boot/efi and flaged as boot, esp as it is described in the Manjaro Guide, which is in the live session and also in the Manjaro wiki.
‘Installation Guides - Manjaro’
The Manjaro guide does not take into account what to do when you have different hard drives. Then, you could try and search in the forum.
‘Questions about dual booting Windows after installing Manjaro - #2 by michaldybczak’
Hope this help, regards