Boot from the "wrong" disk

[samoht]

There are 2 disks in the computer (Legacy UEFI mode):

(/dev/sda) SSD (2 partitions): efi, siduction 18.3.0 Patience - kde - (201805132121)
(/dev/sdb) HDD (2 partitions): KDE neon, data

Installed with siduction/calamares in automatic mode, and then completely updated: no errors.

After a system freeze/hard reset, the computer always boots from the HDD (KDE neon grub menu), although the SSD was selected as start disk in the computer boot menu (here via F11)!
(How can such a thing be explained?)

My real question:
Can I simply rebuild the boot sector on the SSD/efi partition using

Code: [Select]

# grub-install /dev/sda
or is there anything else you need to do?

Greetings,
Tom

Update:
Admins, please move this posting into the English department!

[GoinEasy9]

The first place I would look would be in your uefi bios.  See what order your uefi menu has.  If you don't like the order, you can usually change it there. 

Grub doesn't control which distro comes up first, your uefi bios does.  Check there first. 
Reinstalling grub would only help if it's not on the uefi bios menu. 
Tom

[Sir_Aim]

I had some freezes on my system where the Mainboard/BIOS decided to ignore my SSD on boot. (was still selected as 1st in boot order)

 I had to plug it out, boot once without it and plug it back in.After that I had to select it again as 1st boot disk.

Happened 3 times but always worked.

[samoht]

Thank you both!

It's my friend's machine. I will report soon!

Greetings,
Tom