Check how Windows 10 Pro has been installed as UEFI or Legacy BIOS, which can be done by opening the System Information application found at Control Panel / Administration Tools / System Information.
I terms of the actually installation, I strongly recommend using a second hard drive for the Linux installation, mainly because this will stop each operating systems from overwriting each other’s boot loaders.
But if you want to install both on a single hard drive, you will have to create some space for the Linux which can be done in Windows using the Disk Management application, selecting the disk, choosing the shrink option, specifying the amount of space and click the Shrink button to confirm.
On the list of the devices that appear, you will typically see a 20.04 and USB 20.04 or something to that extent which is important as the USB option is used for Legacy BIOS installation and 20.04 is used for UEFI installations, so make sure to choose the one that matches the installation of Windows.
The installation of Ubuntu 20.04 is fairly self explanatory, but when you but when you get the Installation Type page, make sure to choose the Erase disk and install Ubuntu option, then click Continue.
Under the Select drive, choose the hard drive that does not have your Windows 10 installation on it, you will see a summary of partitions that are going to be installed, so click Continue.
Once the installation process is complete, you will be prompted to remove the flash drive, so do so and reboot the system.
CHOOSING THE OPERATING SYSTEM TO BOOT INTO
By default the next time you turn on your computer, you will be greeted with the GRUB Boot Loader, which now gives you the options of choosing to boot into either Windows or Linux.
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