Can't install Dualboot Ubuntu and Windows10

[Troubleshootpc]

I wanted to install ubuntu on my windows 10 computer via dualboot but while installing ubuntu i got the error "grub-install /dev/sda failed fatal error" after that the computer rebooted and no operating system would load it said error "Reboot and select proper boot device or insert boot media in selected boot device" then I had to reinstall Windows, what should I do?

[CWALD]

First guess is a partitioning issue. Unfortunately the error doesn't give us much other then the drive it failed to install to. The fact that it doesn't list a partition may be a clue however. Try partitioning your drive when you install the first OS instead of later. That way the second one doesn't have to try resizing the first.

[Troubleshootpc]

On 6/9/2023 at 5:21 PM, CWALD said:

First guess is a partitioning issue. Unfortunately the error doesn't give us much other then the drive it failed to install to. The fact that it doesn't list a partition may be a clue however. Try partitioning your drive when you install the first OS instead of later. That way the second one doesn't have to try resizing the first.

The capacity of the disk is 500 gigabytes, the system Windows 10 has allocated 250 gigabytes for the C drive, the remaining 221 are unallocated, but when I try to install Ubuntu during the installation, it shows me 221GB unusable and I can't do anything, it is impossible to allocate partitions. Can you explain more clearly what to do step by step and I will answer you, thanks for the feedback.

[dragonfly43]

Fast boot ? Secure boot ?

You have to disable them all.

[CWALD]

On 6/11/2023 at 6:52 AM, Troubleshootpc said:

The capacity of the disk is 500 gigabytes, the system Windows 10 has allocated 250 gigabytes for the C drive, the remaining 221 are unallocated, but when I try to install Ubuntu during the installation, it shows me 221GB unusable and I can't do anything, it is impossible to allocate partitions. Can you explain more clearly what to do step by step and I will answer you, thanks for the feedback.

Sounds like you already did what I was suggesting. Next step would be to check you bios boot settings, especially those that @dragonfly43 suggested. 

[Atharv]

On 6/11/2023 at 5:22 PM, Troubleshootpc said:

The capacity of the disk is 500 gigabytes, the system Windows 10 has allocated 250 gigabytes for the C drive, the remaining 221 are unallocated, but when I try to install Ubuntu during the installation, it shows me 221GB unusable and I can't do anything, it is impossible to allocate partitions. Can you explain more clearly what to do step by step and I will answer you, thanks for the feedback.

Instead of leaving it unallocated, make an drive then it will surely come up as Usable