I have some problems setting up a dual boot between Win10 and Ubuntu20.04.
To be specific, it seems the bootloader grub doesn't find my second hard drive.
Note: I've added some pictures at the bottom.

My setup:

• Microsoft Surface Pro (2017) with internal SSD (256GB)


• SanDisk - High Endurance MicroSD-Card (128GB)

My Plan:
Set up a dual boot with Win10 on the internal SSD and Ubuntu on the added MicroSD-Card.

What I did:
Note: I've tried to install Ubuntu serveral times now and always ended up with the same grub-problem.

• I used "rufus" to create a bootable USB-Stick. I made sure that UEFI is enabled. (pic. 1)


• I disabled "fast-boot" in Win10 and "Secure-boot" in UEFI


• I booted with the USB-Stick, went into live-mode and installed Ubuntu. On my...
  
  
  
  • ...fist try I chose "Install Ubuntu alongside Win10".
  
  
  • ...second try I chose "something different" and partitioned the drive myself (like in the answer on this question).
  
  
  • ...third try (current state) I chose "erase disk".
    Every time the installation went well, but not once I could boot normally.
  
  
  
  

My Problem:
When I restart after installation I always end up in the "grub bash like" terminal (pic. 2).
I read in a tutorial that I have to tell grub where my "root" is, but when i type "ls", the second Hard drive (MicroSD-Card) doesn't even show up.
When I type "exit", Win10 boots normally. When I reboot, enter UEFI and tell it to boot into Ubuntu directly, it works (pic. 3&4).
But on every "normal" boot, it sends me to the grub terminal.

What I've tried:

• I've tried different installation methods (no difference)


• I've tried using the Boot-Repair tool. No effect.

I've had the same problem a year ago with my main Desktop-PC (I also used a separate SSD for Linux) and had the exact same Problem.
Last time I surrendered. This time I want to find a solution.

Pictures:

1. Rufus - Bootable USB-Stick


2. Grub2 - Bash like terminal


3. UEFI - Boot-configuration


4. Grub2 - Boot-menue

Edit 1:
I tried boot-repair again, this time in advanced mode. These were the settings (those were default)

• Main options


• location

The returned Summaries:

• Info


• Repair

Here is a Picture of the partitions in GParted

New Problem: Now I can't even boot manually into Ubuntu via UEFI anymore. The entry is still there but if i chose "boot with ubuntu" the screen goes dark for 2 sec. and I end up in UEFI again.

Note: I had to delete some pictures because i can only incluse 8 Links.