I have a MacBook Pro ("Core i7" 2.6 15" Touch/Mid-2018 2.6 GHz Core i7 (I7-8850H)) with Windows 10 installed using bootcamp.
I am not sure about the macOS version, but it must be before Mojave. (You will know the reason why I cannot check it now)
I wanted to install Ubuntu (a Linux flavor / varient) as a third OS, attempting to "triple boot".
I followed an online tutorial (https://robpickering.com/triple-boot-macbook-pro-macos-windows-10-linux/).
First (Subtitle in the tutorial: "Create Partition for Linux"), I needed to create a partition for Ubuntu by shrinking the volume of the bootcamp partition, which had enough space, in Windows OS.
Unable to do it through the inbuilt Windows disk management tools (diskmgmt.msc), I used a third-party software EaseUS Partition Master 13.0 following another online tutorial (https://www.easeus.com/partition-manager-software/windows-cant-shrink-volume-partition.html) and succeeded.
Then (Subtitle in the tutorial: "Install rEFInd"), I was asked to disable macOS's System Integrity Protection (SIP) in order to install rEFInd (a Boot Manager), by booting into macOS's Recovery.
Now this is the crux:
I rebooted the computer and held down Command-R as instructed.
The screen only showed a spinning globe and I was asked to pick a network. After some research, I believed I launched into Internet Recovery instead of simply Recovery. So, by holding down the power key to reboot, I retried multiple times and still got to the same screen.
Note: At this point, I was still able to switch between macOS and Windows 10 and run the OS normally by holding down Option during booting.
Having failed continuously, I decided to give Internet Recovery a try.
Due to limited choices, I selected a quite-stable-but-not-too-stable private wifi as the network, and just let the globe spin.
After about 30 minutes, the globe was still spinning and nothing showed up. I decided to reboot the computer by holding down the Power key.
Everything is still normal.
Then I repeated the same thing but for about 1 hour, still to no avail and similarly forcefully rebooted.
Held down Option key while rebooting.....OH MY GOD! I lost the option to boot into the macOS.
In the selection screen, there is only Windows 10 available, but with no other choices!
After some time calming down and attempting to find solution (which did not succeed), I decided to launch into Internet Recovery but this time REALLY for recovering my macOS.
I waited 10 hours.
Still a spinning globe.
At that moment I had jobs to do and I could only forcefully shutdown the machine.
I would like to ask for suggestions of the best things to do now, in order to save this mess.
If there is any method, I would also like to know whether my data on macOS and/or Windows 10 would be safe, and which step is risky, which step is not.
Due to physical constraints, I may not be able to find a stable wifi. A wired household network may be available.
I have a spare USB (16GB) and SD-card (128GB). By "spare" I mean empty.
I can still only go to Internet Recovery but NOT Recovery.
Not having to visit Apple Store or buying new thing is preferred, and I also do not want to mess up with my friend's MacBook.
But all advice and explanation are welcomed and appreciated, so please feel free comment if you come up with any ideas!
Thanks in advance. I would be very happy if you could help! :)