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I have two hard drives, one with Win 10, one with Ubuntu. I can change the OS by going into Uefi Bios and selecting the disk as the boot drive. However, I would like a boot menu so I can select after restart. Please help.

Jake B.
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    I suggest you read https://en.wikipedia.org/wiki/Power-on_self-test , https://en.wikipedia.org/wiki/BIOS , https://en.wikipedia.org/wiki/Master_boot_record , https://en.wikipedia.org/wiki/GUID_Partition_Table , https://en.wikipedia.org/wiki/UEFI . One will have the Answer. Read the others to understand that one. – waltinator May 24 '21 at 18:19

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If grub menu is not showing up for you when booting from ubuntu drive you can try force it to update as explained in this question:
https://unix.stackexchange.com/questions/49989/let-grub-re-scan-disks/49998

but a short summary is:
run

sudo update-grub

and you should be getting os selection when booting linux.

Bellator
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    You have to have both systems in same boot mode, probably UEFI. And Windows fast start up off. Grub only boots working Windows which means fast start up/hibernation must be off and Windows does not need chkdsk. http://askubuntu.com/questions/843153/ubuntu-16-showing-windows-10-partitions & https://askubuntu.com/questions/145902/unable-to-mount-windows-ntfs-filesystem-due-to-hibernation – oldfred May 24 '21 at 19:13
  • good to know, for me generally this always works.
    @Jake B. if update-grub does not work check if bots systems are in same boot mode :)     – Bellator May 24 '21 at 19:43
  • This doesn't work for me consistently (update-grub). I have fast start and hibernation off. What happens is: I have disk A (Win) and disk B (Linux). I set disk B as the default boot in bios. Then GRUB menu loads and I can boot to Linux, but not to Windows ("No such device", "File /efi/Microsoft/Boot/bootmyfw" not found. So I can only boot to Windows if I press F8 and select Win disk in bios to boot from. If I again restart, GRUB shows and I can boot to Win. But if I select Linux again, then restart, next time I cannot boot to Win. Please help, this is furstrating. – Jake B. May 27 '21 at 13:59