mjhjmtu

First of all I have dual boot Win10 + MX Linux (was Ubuntu like 2 days ago).
I have two disks, both systems are on /dev/sda (which is SSD, and parted -l for this disk is below)

Number Start End Size Type File system Flags

1 1049kB 106MB 105MB primary ntfs

2 106MB 80.6GB 80.5GB primary ntfs //win10

3 80.6GB 81.4GB 824MB primary ntfs diag

4 81.4GB 128GB 46.6GB primary ext4 boot //linux

Let's talk what have I done in the past to my set up.

I use legacy mode (UEFI and secure mode disabled) - because in the past ubuntu didn't want to boot, GRUB is on /sda4 (boot flag?)

I used "recovery CD" with Win10 because I had the "missing bootmgr" message on the win10 boot - I think that's when /sda1 got created (and on that partition I can see bootmgr file).

Lately my linux sometimes does not want to boot. Usually when I start PC for the first time it won't boot (like always) but on the nd/rd try it will boot.
There is just _ cursor blinking, I can't access tty (ctrl+alt+f1). I thought this is related to

a) grub - tried to repair it - nope

b) something with system - switched from ubuntu to MX linux - nope

Win10 boots fine.

So my question is what can I do to make boot consistent?

And what are these partitions: sda1 and sda3?

Should I switch to EFI? (if so how to do that safely)