Fix Persistent USB

I had Manjaro KDE stable installed with Windows 10 as dual boot on my HDD, then I wanted to install Manjaro KDE dev (Unstable branch with KDE Unstable repo) to my USB, during the process I selected to install grub on my HDD but in the end I found that the grub appears only if the persistent USB is connected which means that it’s effectively installed on my USB, so I followed these instructions to reinstall grub on my HDD, in the end the grub was restored and I can see it without needing the USB to be plugged, but I can’t boot the installed OS from my USB even if I select boot from USB in BIOS, is there any steps I missed ?

Why would you want to boot from USB
when your boot loader (Grub) is installed to your internal disk?

… and explicitly NOT to your USB, as you say …

as long as the USB disk is connected, it should be possible to boot it, or any system on any currently connected disk

but I may be missing something important here

The USB will be used only from time to time to test new cutting edge updates from Manjaro Unstable Dev branch (especially new git KDE Plasma features), so my main operating systems (Manjaro Stable + Windows 10) are installed on my HDD which requires the grub to be installed on HDD.

If the boot loader is installed on the HDD
as opposed to the USB
it should be able to boot any system off of any connected drive
your USB drive included
No need to (via BIOS) try to boot from USB.

When you install and update grub
in my mind it should be enough to have that USB connected - so grub can pick up it’s presence and add a boot entry for it for you to choose from

… it’s not so much where you install grub to
it’s what it can see to be present at the time you configure/install it