I installed rEFIND Boot Manager, got mysterious Legacy OS thing that doesn't work
You can follow this tutorial even if you don't know how to use a terminal.

If you don't want to scan legacy operating systems, do the following:

The reason this is happening is because rEFInd scans for legacy operating systems as well by default. These steps will help you disable legacy OS scan.


Open Terminal. Search for it in spotlight.
Type diskutil list, press enter and identify your EFI partition. By default it is disk0s1, but if it is different for you, then you should use the right identifier.
Type diskutil mount disk0s1 and press enter. The EFI partition should be mounted now.
Type cd /Volumes; ls and press enter. A list of mounted partitions will appear. If you see "EFI", then continue the tutorial normally. If not, look for "ESP", "Untitled" or "Untitled X" where X is the highest number possible. Use that for the next step instead of "EFI".
Type cd ./EFI; ls and press enter. From the list, find the folder that has the word "refind" in it, and use it for the next step. It can just be "refind" too.
Type cd refind; ls where you replace "refind" with the right folder name if it is different. You will now see the files inside the rEFInd folder. Search for refind.conf. If you can find it, type nano refind.conf. If you can't find it, then there is a problem and you should reinstall rEFInd.
You will be presented with a minimal text editor. Press Control+W, type scanfor and press Enter. It will find the nearest scanfor. If the cursor is at the first or the second character of a line now, continue to the next step. If not, repeat this until the cursor is at the beginning of a line.
If the first character is #, press Backspace once. 
Using the Right Arrow Key, move the cursor to the end of the line, and delete everything with Backspace key except scanfor and the space after it.
Type the following: internal,external,optical,manual.
Press the following key combinations one at a time: Control+o, Enter, Control+X.


Try rebooting now. Legacy should be gone!

(Edits are appreciated, I might not be clear enough)