GRUB install fails

Hi

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