How to install another distro alongside Ubuntu Mate and Win10

[anon42388993]

Is that hard drive unassigned? Not formatted at all?

We also have a installation guide that may help. I do not know if you have seen it.

Ubuntu beginners guide, complete how to install and run first update

WARNING 1: BACKUP ALL YOUR DATA BEFORE YOU START TO INSTALL IN CASE THINGS GO WRONG!. SAVE ALL YOUR IMPORTANT FILES AND FOLDERS TO EITHER AN EXTERNAL USB DRIVE AND/OR TO A 2ND HARD DRIVE ON YOUR PC. DO NOT SAVE ANYTHING ON THE SAME DRIVE YOU INTEND TO USE FOR LINUX UNLESS YOU UNDERSTAND/KNOW WHAT YOU ARE DOING!. ONCE YOU START PARTITIONING AND YOU COMMIT TO FORMATTING YOUR PARTITIONS, THERE IS NO GOING BACK!. WARNING 2: IF YOU HAVE A RECOVERY PARTITION AND HAVE NOT CREATED ANY RECOVERY DISKS FO…

I am not familiar with EFI and cannot help, but you may find this helpfull.

https://help.ubuntu.com/community/UEFI

https://help.ubuntu.com/community/UEFIBooting