Hello Good Peeps 
I would like to know how I can install another distro (Mint 17.3) alongside Ubuntu Mate and Win10. I have a separate disk set aside for mint 17.3. When I try to install mint it gives me the option to install alongside Mate (and only mate - doesnt mention win10) but I cannot choose to install it to a different drive. I am a bit afraid of just trying things out as I have some pretty important stuff on both my win10 and ubuntu mate drives.
Help would be much appreciated.
Luke
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.
I am not familiar with EFI and cannot help, but you may find this helpfull.
Hi Luke,
further to @anon42388993’s advice, see the partition guide which is also linked in the beginners guide above!:
Thank you both. @anon42388993 the drive is brand new and unformatted. I will follow the instructions and format it accordingly. It looks like I have some reading to do … 
Thanks !
Ok, this looks way harder than I thought it would be. I think that having two linux operating systems and win10 is a bit too complex for the average person like myself
Its a pity tho, as now I am made to chose between Mate and Mint – I will obviously stay with Mate, but would love to give Mint a go on hardware …
There is hardly any difference between Mate and Mint imo, the installation process is the same and really it comes down to how you want to set up your installation as far as partitioning goes, you can auto install if you like and just let the installer take care of the rest, this point is covered in the beginners guide and you just need to tell the installer how space you want to use for Ubuntu Mate (Please be aware that this option will only create a swap and combined root and home partition!):
So I found the courage and installed mint alongside mate and win10. It turned out to be not so hard after all. All I had to do was create the partitions on the new disk as outlined in the articles @anon42388993 and @wolfman suggested, install mint and directed mint grub to an external disk (so that it wouldn’t mess with the ubuntu mate grub). Then when I booted in (with the external disk unplugged) and I logged into ubuntu mate and updated ubuntu grub, which found mint without a problem.
There is probably a smarter way of doing this, but this worked