I added more hard drives to my windows PC to dual boot as a test, then added another drive to actually play since I was enjoying Linux. My third OS isn’t bootable any more. What have I done wrong?

Started off with Windows 10 on a SATA drive with an M.2 drive for more data.

Added a 2Gb NVME with Debian - this has become my daily driver. I haven’t been to windows more than a few minutes a week.

Added another 250Gb SATA drive to test and play with another Debian install so I don’t break my daily driver.

Tried today to boot into the test OS and It’s just missing from GRUB? It doesn’t show up as a bootable drive in my UEFI BIOS either, though the drive itself is seen.

From KDE Partition Manager in my daily driver Debian, the drives are:

/dev/nvme01 - the daily driver Debian

/dev/sda - the Windows OS drive

/dev/sdb - the windows M.2 drive

/dev/sdc - the test Debian drive (not booting)

I would appreciate help. While there’s not much on that drive, I would like to continue my playing around.

Thanks in advance.