GRUB install fails

I have an issue with writing GRUB to /dev/sda and /dev/sda{1,2,...}

I have GRUB on a USB pen, so I can boot, but it is not the reccomented way.

I have a labtop with partition table as listed below, it contains:
- Pre-installed Windows 10 (acer) that can boot using UEFI (on partition 1, 2, 3, 4)

- Accessable in UEFI secure mode (donøt know how to disable secure boot in "BIOS")

- An linux Mint on partition 5, installed in Legacy mode

- An Debian with /boot on partition 6 and the rest on partition 8 (LUKS)
- Installed in Legacy mode (A fresh debian 8.6)
- Afterwards upgraded to stretch

- Upgraded grub to grub2 (see grub progrs below)

- Unused partitons: 7, 9, 10

I would like to modify the GRUB menu a bit and I have changed the grub config files, executed "sudo update-grub" and it looks fine.

sudo grub-install /dev/sda
Installing for i386-pc platform.
grub-install: warning: this GPT partition label contains no BIOS Boot Partition; embedding won't be possible.
grub-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.
benny@bygdagrovi:~$ sudo grub-install /dev/sda7
Installing for i386-pc platform.
grub-install: warning: File system `ext2' doesn't support embedding.
grub-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.
benny@bygdagrovi:~$ sudo grub-install /dev/sda10
Installing for i386-pc platform.
grub-install: warning: File system `ntfs' doesn't support embedding.
grub-install: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.

I have tried to format my spare partition (7 and 10) to various fie systems in gparted, but could't find a good one.

Partition layout

sudo parted -l /dec/sda
Model: ATA LITEON CV1-8B256 (scsi)
Disk /dev/sda: 256GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number Start   End     Size    File system Name                          Flags
1      1049kB 106MB   105MB   fat32        EFI system partition          boot, esp
2      106MB   123MB   16.8MB               Microsoft reserved partition msftres
3      123MB   26.5GB 26.4GB ntfs         Basic data partition          msftdata
5      26.5GB 34.5GB 8000MB ext4
6      34.5GB 35.5GB 1000MB ext4         dboot
7      35.5GB 36.5GB 984MB   ext4         extra_boot                    bios_grub
10      36.5GB 36.5GB 16.8MB ntfs         biosmbr                       msftdata
8      36.5GB 247GB   210GB                luks
9      247GB   256GB   9024MB ext4         exchange
4      256GB   256GB   524MB   ntfs         Basic data partition          hidden, diag

Grub programs
dpkg -l|grep -i grub
ii grub-common                                   2.02~beta3-1                         amd64        GRand Unified Bootloader (common files)
rc grub-legacy                                   0.97-72                              amd64        GRand Unified Bootloader (Legacy version)
ii grub-pc                                       2.02~beta3-1                         amd64        GRand Unified Bootloader, version 2 (PC/BIOS version)
ii grub-pc-bin                                   2.02~beta3-1                         amd64        GRand Unified Bootloader, version 2 (PC/BIOS binaries)
ii grub-splashimages                             1.2.3                                all          a collection of great GRUB splashimages
ii grub2-common                                  2.02~beta3-1                         amd64        GRand Unified Bootloader (common files for version 2)
ii grub2-splashimages                            1.0.1+nmu1                           all          a collection of great GRUB2 splashimages

It is not a requirement the I boot in legacy mode, if it is better to switch to UEFI I would like to do that (but don't like to disable booting into MS Windws (and it is not reconized by update-grub).

Any hints?

Br,

Benny