after researching from various different sources, I managed to learn a bit about the boot process, the BCD store and grub. I found a solution.
I learned that grub mbr, the sort of stage 1 grub was installed on the main internal disk (C:). however stage 2 was written on the external one (say E:). Grub was unable to find its stage 2 when E was disconnected and therefore the error. now:
1.typing exit assured me that windows boot manager was ok.
2. on the cmd, I typed bcdedit /set {fwbootmgr} displayorder {bootmgr} /addfirst and that made the windows boot manager appear before the grub. I must say that before that I tried bootrec /fixmbr & bootrec /fixboot & bootrec /rebuildcbd But that was unhelpful, as it didn't touch the grub installation.
Now the surface boots directly into windows. But thats not the full solution, since I want linux as well. I actually understood that I need grub on my C drive since surface is unable to boot from an external SSD without some hard work done. My mistake was apparently, that I chose to install the grub files on the external disk. The solution should be re-installing it on the main disk, so I would be able to see the menu, hopefully it will let me choose between linux/windows when E: is connected and windows when its not.
that belongs however to a different topic. I am lazy as for now :) (could just leave it be, typing exit when I want windows to boot. but sometimes I am without the keyboard, using it as a windows tablet).
