Hello there. I tried searching around the forums for a bit and also found seemingly no dupes with the editor's 'check if posted' feature, so here I go.
I own a mid-2012 MacBook Pro ("9,2"?) currently housing macOS Mojave 10.14.4 and Windows 10 Pro (prior to this installation of Ubuntu MATE 18.04.2 LTS), and the two worked just fine. However, after I tried installing Ubuntu MATE by referring to this answer on triple-booting (except that it's "HFS" against my current "APFS", so I kinda followed suit), I ended up with four choices in the Startup Manager from left to right: "Windows, EFI Boot, MACINTOSH HD, EFI Boot". Choosing the former EFI Boot (not bootable USB drive) instantly restarts my laptop, while the latter EFI Boot is the Ubuntu MATE I had just installed and added an icon on.
When I choose the Windows one, the whole thing gets stuck at the "Missing operating system" message, but my macOS and Ubuntu seem to work fine. I have tried Boot-Repair's 'Recommended Repair', and what I got was this message:
https://imgur.com/a/n7BlWR8 (NOTE: vertically long)
now I have GParted installed in my Ubuntu, but it somehow reads my laptop's HDD capacity as 465.76 GB, as opposed to the actual 500 GB. Putting that 34 GB difference aside, I don't know where to go from there since I'm not knowledgeable with disk/partition stuffs and this is practically my first time using Ubuntu. I had screwed up big time trying to get the vanilla (non-flavor) Ubuntu coexist in my laptop too, so I started over and I'd like to feel much more assured this time. How do I get rid of my Windows' "missing operating system" message from this point, and be able to get all three OS'es boot up perfectly anytime I want through MacBook's Start-up Manager? Feedback is much appreciated. Thanks for your attention.
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here is the Pastebin report generated by Boot-Repair:
http://paste.ubuntu.com/p/gr9H3pBMZF/
and here is a link to a few photos of the disk's insides and whatnot in macOS's Terminal, including descriptions on each photo:
https://imgur.com/a/ysqfjmQ
if there's anything inappropriate/incomplete in this post, please let me know in this thread and I'll edit/delete it as soon as I get on here again. Hopefully moving won't be necessary...
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