Ubuntu installed alongside Windows on 2nd drive doesn't boot
Have you tried to disable the Windows Fast Startup function?
This has caused me the same problem in the past. Try running this command from the Windows cmd Prompt:
Shutdown.exe -s -t 00
It will shutdown windows completely and if you still don't get the grub menu, then try setting grub as the boot manager for Windows with this cmd:
bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi
Try disabling secure boot in your computer’s BIOS. If you didn’t install Ubuntu with an EFI partition, you may need to enable the legacy boot option in BIOS.