No bootable disk error - help needed!

Got a question for the experts here. My hard drive (HDD) started acting up, so I figured it was time to ditch it for a new SSD. Right now, my setup has two drives: an SSD where all my programs live, and an HDD just for storage.

Here’s the thing. It started throwing errors saying it couldn't read the D drive (that old HDD), and then after a reboot, it told me there was no bootable device. That makes zero sense to me. My boot drive is the C drive (the SSD), not the D drive where the data sits. I checked the BIOS, verified the boot priority, and yeah, it's set to C.
So, I decide to swap out the old HDD for a fresh SSD. I plug it in—nothing. No bootable disk. I put the old HDD back in and everything works fine. I take the old HDD out entirely, leaving just the SSD with my programs, and suddenly... no bootable disk again.
What the hell? How can a drive that only holds data be required for booting?

The rig is a Dell Inspiron 15, running a 128GB SSD and a 1TB HDD...

urbanwalker2 said:... and when I rebooted, it said there was no bootable device. That doesn't make sense, because the boot drive is C (SSD) where the apps live, notD, which holds my files. ...

?

If those 'boot_stuff' (bootmgr etc.) files aren't sitting in the Hard drive (C: e.g.) directory,
then they should be tucked away in some other folder within the macOS root directory
like C:/Windows/Boot/PCAT, for instance.
Or maybe in
C:/Windows/WinSxS/...

But, where those files actually reside (they all belong on a single drive—specifically the one running the OS) isn't your concern...

...unless you manually moved them to different folders or drives based on your own idea of
"how things should work"... ...somewhere among the data(?)...