Solve while still in the installer
I was able to solve this issue while installing Debian Stretch via USB stick on a Dell Precision 5820 with drive encryption. I was not dual booting Windows, but I believe the solution would work if I was. I had already disabled secure boot. I was getting the same error about installing grub in dummy while installing, but I did not exit the installer.
I believe the issue, was where
/boot/efi were mounted. I solved the problem during the install by going back to the partitioning step and changing my partition table with manual partitioning.
First setup boot partitions unencrypted:
- EFI System Partition (ESP) with
/boot/efi mounted - This is required for UEFI boot systems
biosgrub partition, or Reserved BIOS boot area - This becomes space for the bootloader files. You will not be able to make this partition "bootable" because the ESP is bootable.
It is imperative that these be unencrypted, and it is recommended that they are at the beginning of the disk or available free space with other partitions following.
Set up other partitions and encrypt if you want
If you are dual booting Windows, your Windows partition should show up during manual partitioning, and you should not have to change anything for it. I recommend following Debian's recommended partitioning scheme for your remaining partitions.
You may have to clear the data on the partitions (other than your Windows partition!) for this to work, and go through the installer steps between partitioning and installing GRUB again so that the installer puts files in the correct location. If you proceed right to installing GRUB again, this solution may not work.
Hope this helps!