Yes, that was my hypothesis.
That said, i am a little lost at your last explanation, so i will try to analyze it sentence by sentence.
The drive with Manjaro (sda) has /boot/grub, but is not seen by BIOS setup, but is seen by boot menu.
I suppose you mean “boot menu”=“grub menu”
You have to imagine it as a tree or a train First comes your bios (you are actually stuck here if i understand right). Then, in your case, the bios tries to boot from the selected drive. Since it is a MBR based drive, it looks at the MBR (Master boot record, that is). Here it should find the first stage of GRUB, the default manjaro bootloader (GRUB=Grand Unified Bootloader). If that succeeds, the first stage of grub takes the control and tries to load the second stage of itself and the config, which is on a particular partition, usually where the OS is (/boot…). According to its config, it can try to load another OS. So this is actually the very last thing that happens.
Basically, you can fix the things in 2 ways: reinstall grub in its 2 stages on MBR and the partition (grub-install), and reconfigure it after it is installed, to look for another OS on another drive, or boot the current os with new kernel or parameters (update-grub). Untill recently when updating it automatically tried to scan for every possible OS and add id to the menu (like Debian or Windows from the other SSD), that setting is not disabled by default but can easily be reenabled (but you have an old install so it is turned on in your case).
You can now understand why this
Manjaro was installed while the Debian drive was disconnected. So, it is likely the Manjaro grub was created without the knowledge of the Debian drive.
does not make much sense to me. If there was only one drive at install time, grub should have copied itself to the MBR (what you can do with the command above just in case). And the bios should have seen it. On the other hand you are right - it could not have possibly created a menu entry for debian. But, on the first update-grub after it could see the debian disk, it would have created the menu entry. And we have kernel updates which update the grub once a month.
BTW, debian uses the same bootloader, GRUB, so it works the other way around too.
So if the first idea fails (i still have no real idea why the bios does not see the other drive), you can alternatively try
update-grub from debian. Or as a last resort what Gavenga proposed, which is essentially the manual creation of the boot entry in the menu, in case update-grub does not do it automatically.