I have an old system that I built myself in 2010. The motherboard is an MSi x58 Pro-E (MS-7522). It has two 240 GB SSDs, one with Windows 10, the other had Ubuntu 20.04.6 LTS. I set the BIOS to boot into the Ubuntu disk and used grub to let me decide between the two OSs. Both Ubuntu 20.04.6 and Windows 10 were working fine before I started this adventure.
I had partioned the Ubuntu disk, but root was too small for updates over the years, so I decided to install the latest Ubuntu. I downloaded ubuntu-25.04-desktop-amd64.iso 5/1/2025 and used balenaEtcher to burn it to a usb stick. Rather than create the partitions myself, I told the installer to write over my old installation and use the entire disk. All seemed to go well, but when I try to boot up the system, all I get is a blinking cursor in the upper left corner. If I change the boot sequence in the BIOS to start with the other SSD, it boots just fine into Windows 10.
Using the usb stick, the installation looks like it went okay. Here are the partitions the installer created:
Device Start End Sectors Size Type
/dev/sdc1 2043 2203647 2201600 1G EFI System
/dev/sdc2 2203648 468858879 466655232 222.5G Linux filesystem
I suspect this has to do with my motherboard not supporting UEFI, but I don’t know how to fix that.
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1st important question; when you installed Windows 10 did you do so in legacy BIOS mode or did you use UEFI mode as it would have been if you had purchased the computer with Windows pre-installed?
EDIT
I’ve just noticed the MB is legacy BIOS only!!
2nd question following that is how did you install Ubuntu? I think it is probably in UEFI if you let the system install using the default method.
Both OSs must use the same mode to allow dual booting from grub and I suspect this is the reason behind your problem
Perhaps a reinstall of Ubuntu ensuring you choose BIOS, not UEFI is going to be the quickest way out of this but the best way to get all the info we need is to run the Boot-Info script, part of Boot Repair.
See Boot-Repair - Community Help Wiki
Show us the link you will be shown and it will tell us a lot, hopefully allowing us to tell you how to get back to your dual boot using grub
I don’t see any way to tell the installer to use BIOS instead of UEFI. I tried using the manual installation. When I removed the /boot/efi partition, the installer popped it back in. Seems odd that the installer wouldn’t detect that my machine doesn’t have UEFI and act accordingly. I recall booting a Data General Nova 3 from front panel switches in the middle 1970s. Sigh.
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The mode the installer boots is the install mode. The Ubuntu ISOs boot both ways (well not on your legacy MB) depending upon the BIOS/UEFI Settings, but some media creation tools will allow you to select only one mode if you want. The installers may(?) just put the 1GB EFI by default these days – way to big for some of my small systems, so I have to make the EFI before the install if I want it smaller. Maybe no way to get rid of it completely! It is possible to boot a legacy system off a GPT partitioned disk,but you need a grub-bios 2MB raw partition for some boot code – not needed if yout stick to MSDOS partitioning.
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I ran boot-repair and got this, http://paste.ubuntu.com/p/fWGdKnQrys/
My Ubuntu 25.04 is on /dev/sdc1 and /dev/sdc2.
There is NO reason not to install BOTH UEFI and CSM bootloaders.
But Ubuntu insists in installing only one (which it thinks is the correct one).
In the result, Ubuntu fails in many variations.
In my personal case, it fails exactly in the other direction:
Both Ubuntu Desktop and Server install CSM boot on my computer, do not offer option to install UEFI bootloader, and so Ubuntu boots in CSM, losing the functionality of UEFI.
So this thread shows : Epic Ubuntu fail!
Edit: this is one of the reasons I am writing an inofficial installer for Ubuntu who does just this and more things that the Ubuntu ZFS installer is not capable of…
According to line 101 from the boot-repair report
The firmware is EFI-compatible, and is set in EFI-mode for this live-session
When you boot the installation USB disk, do you see two boot options for this disk in your PC boot menu?
- UEFI: Name of disk (e.g. Sandisk/Kingston etc)
- USB: Name of disk (e.g. Sandisk/Kingston etc)
Picture attached to illustrate UEFI and Legacy boot choices
Your Windows disk (sdb) is not GPT (line 122)
Your Ubuntu disk (sdc) is GPT (line 123)
If your PC is actually UEFI compatible, then Windows should be installed in UEFI mode with GPT.
To reiterate previous replies, both systems should be installed in the same mode.
Finally, before you do anything else, please confirm that you have backups of both your Windows data and Ubuntu data?
I got it to work. I changed the boot order in my BIOS so it booted directly from the USB stick, and Ubuntu installed just fine. I’m writing this on my new install.
I’m not sure what went wrong earlier. I think that I burned the iso file to the USB stick instead of burning a disk image. So I had to use F12 when booting to bring up a dialog that allowed me to do a USB boot. That apparently always installs as though you have UEFI hardware, which my machine does not.
I very much appreciate the time you all took to help me think through things more carefully and come up with the right way to do things.
And, yes, tea-for-one, I did back up everything that was significant–and probably a bunch of crap that I’ll never have any use for. 
I’m a retired software engineer and teacher. Been doing this stuff since the early 1960s. Got a few stories about not backing things up. Value of backup = cost of redoing everything you’ve lost.
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