Please bear with me .

Okay, this is a new box with an Gigabyte GA-AX370-Gaming K5, Ryzen 5 1600, DVD, old harddisk with two partitions and a m.2 SSD. Tumbleweed is installed on the SSD (via USB port). Booting is in legacy mode, not UEFI.

Since this is for my playing son, I wanted to (re)-install Windows 7 on the hard disk which contained originally a broken Win7 installation. So I changed the booting sequence in the BIOS and set DVD first medium.

In spite of that the broken Win7 of the hard disk tried to boot (and crashed). I mistakenly assumed that Win7 was the culprit, so I re-partitioned and re-formatted the hard disk to get rid of the broken Win7. Booting now from the DVD results in a blinking cursor on an otherwise black Screen.

In my understanding the BIOS refuses to load the OS from the DVD and accesses the hard disk. How can I convince it otherwise? Thanks for any suggestions .

When you start to boot there should be an option to go to the BIOS or go to the boot screen & choose what to boot from, I'd try that.

I do. To be exact, I press F12 or I switch the boot sequence in the BIOS and try to boot then -- both with the result I described above. To reiterate: the machine is requested to boot the DVD but it is still accessing the hard disk.

Have you tried booting off the same DVD on another computer?

Yes, it works.

I've had a problems in the past with a BIOS that required me to turn off a "Quick Boot" or "Fast Boot" setting (not the Windows 10 concept of Fast Boot) - have you had a look in your BIOS settings for something similar?

Disconnect the disk and ensure the box will boot the DVD ok. Then use dd to zero out the boot code in the MBR (presuming your BIOS reference is correct).

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@hydrurga Yes, there are even three settings off / fast / fastest (or some such). It is set to off.

@syg00 Good idea. I will try that tonight.

Question: The SSD with tumbleweed is GPT formatted. Does that mean that the machine does an UEFI boot and might that be the problem? The BIOS else shows "legacy" wherever I could chose that.

A Linux system can use a GPT drive with a BIOS boot partition and not need UEFI. My understanding is that with windows, if you have a GPT drive you must install it UEFI. Since your original system was windows 7, it was in all likelihood a Legacy/MBR install. Since you are trying to install windows to another drive which is not GPT, this should not be a problem. If you have UEFI and Legacy/CSM options, you should have it set to Legacy when trying to install windows 7.

Did this computer originally come with a pre-installed windows 7?

Remove hard drive and see if it boots.

It was a bit of an issue for me trying to get windows 7 to load on a new uefi board. Stupid usb problem. Gigabyte may offer a converter for that board to use a full retail windows 7 to usb and it adds in usb drivers.

@yancek I guess the hard disk was the system disk in the old computer. With the exception of this hard disk (and the DVD drive) all other components are new.

Urgh. I disconnected the hard drive. Now I get

When I press a key, the message repeats.

Any ideas?

You've perhaps answered this already, but have you tried booting off any other bootable DVD (distro, GParted Live, etc.)?

No, I haven't. I can try later but the real boot process doesn't start at all as far as I can determine. The box is just accessing the wrong device...

But the box normally boots from your USB connected m.2 ?. That's odd.
If t'were me I'd just boot the Linux and zap the MBR, but I'm concerned about the reference to "legacy mode" - that suggests a [U]EFI m/board. Who knows what release, and what Win7 needs ...

I guess if it's already broken, maybe not much to lose by experimenting.

That is a misunderstanding, the m.2 has a special / dedicated slot in the mainboard, nothing to do with USB as far as I can discern: https://en.wikipedia.org/wiki/M.2

Sure, I can "zap the MBR" (of the disk I want to perform the install on) but the DVD has to be accessed to read the data for installation.

Right, I just checked, there is a BIOS-Version of 8/8/2018 while mine is currently 4/17/2018 perhaps I should flash that...