Disconnecting all other disks apart from the disk you are installing an OS to, is always recommended; this ensures no complication (example; writing to the wrong ESP) because other disks are then not present during install.
Reconnect disks only after installation has fully completed.
Manjaro will always install GRUB; (AFAIK) there is no option not to, as there is with some other distributions. Make sure os-prober is also installed and activated (OS Prober is the mechanism that detects another OS during boot).
Find further information on os-prober and other multiboot caveats in my article:
And it can get even more confusing as not everyone speaks/understands English natively. Of course it doesn’t take Einstein to appreciate that, but it gave me an excuse to use the quoting as an example.
Simply select text you wish to quote, and the quote button should appear immediately above the selection. Click quote, and the selected text appears in your post wherever the cursor happens to be. Cheers.