SSD: sda1 EFI fat32 512MiB | sda2 / HDD: sdb1 /var | sdb2 swap | sdb3 Downloads | sdb4 Old Windows MBR NTFS 512 MiB | sdb5 Windows C HDD: sdc1 /home | sdc2 Windows Data RAMDISK: /tmp, /run, /run/lock
All partitions tables are GPT. Both OS use EFI and GRUB boot from sda1. sdb4 is not being used.
Q1: What I want is to disable GRUB menu (only appear when pressing shift or another key) and choose my OS pressing the BIOS select device key (F10).
I tried formatting sdb4 to FAT32 and copy windows EFI files there, then from GParted flagged as boot, and esp, then updated GRUB config and run
update-grub. The attempt failed, I'm unsure if I have to move sdb4 so that becomes the first partition on sdb and if windows requires something else to "set the partition as Active", from my understanding the flags boot and esp make the same effect (as Windows "Active"). The error I got was "no bootable device found". I could revert to my previous setup using Parted Magic and then running Ubuntu in recovery mode.
Q2: I would like to know if there is any disadvantage to using my BIOS device menu over GRUB, I believe GRUB is more useful while having 1 disk and many OSes.
I don't want just a copy-paste answer, I want an answer that takes me through the steps so I can understand what the solution is doing.