[ILUG-BOM] Help required in installing Ubuntu alongside Windows 10

Hi,

I have a Windows 10 PC with was installed in 'legacy' mode (not EFI).

I shrunk my C: to create free space of 25Gb for Ubuntu.

I created a 64 bit Ubuntu bootable USB pendrive.

For booting from the pendrive, I select the boot mode at the BIOS splash screen I have tried both 'USB drive' and 'EFI-USB-drive'.

I tried to install Ubuntu 15.1 but the free space is always marked 'unusable'.

Before this I'd got a warning that the installation was started in EFI mode, but target installation is non-EFI, and if I force it I won't be able to switch OSs. But this warning is not appearing subsequently, only my freed partiton is showing 'unusable'.

Is there a way install Ubuntu into the created free space in such a situation?

Thank You!