iMac Bootcamp and Magic Mouse woes I've got into  a right pickle trying to get my Bluetooth Magic Mouse working with my bootcamped iMac. It works fine on macOS, but in Windows it currently isn't scrolling, right clicking and periodically won't connect on start up and I'll have to go into macOS, disconnect and reconnect to get it working.

Tech Specs

System Software Overview (from macOS):

  System Version:   macOS 10.13.3 (17D47)
  Kernel Version:   Darwin 17.4.0


System Info (From Windows 10)

OS Name Microsoft Windows 10 Home
Version 10.0.17134 Build 17134
Other OS Description    Not Available
OS Manufacturer Microsoft Corporation
System Manufacturer Apple Inc.
System Model    iMac16,2
System Type x64-based PC


Things tried (and how we got here)

It started off with just the not connecting at all when booting into Windows, in spite of everything I tried from the Windows side. Trying to connect to the mouse through Bluetooth repeatedly failed, and turning the mouse off, on, clicking, etc, all the normal things which might fix this normally, didn't.

What follows is the edited highlihts of my attempts to fix this, with much flailing, swearing, restarting, swearing some more and storming off omitted. This is not a comprehensive list.

Reading at this stage included:

Apple Magic Mouse is discoverable, but not pairing (doesn't seem to apply here, even though I tried this)

https://discussions.apple.com/thread/7151690 (my version of Windows isn't even listed here and the download link is quite old). Before posting this, I decided to try this link, and am told using the newest version that "This version of Boot Camp is not intended for this computer model.".

https://discussions.apple.com/thread/8175827 (from this, I unchecked allow this computer to turn off the device. Helped, but didn't fix entirely)

https://answers.microsoft.com/en-us/windows/forum/windows_10-start-winpc/windows-10-not-pairing-with-apple-wireless-mouse/b3444d1e-2c87-4760-b329-fe0ed3b016b5 (hardware troubleshooter useless, Windows Update had nothing, and this computer's had more restarts than I can even count)

https://discussions.apple.com/thread/7439976 (temporarily solved the issue each time for a few restarts)

https://discussions.apple.com/thread/8125747 (tried resetting the SMC and the NVRAM to see if it helped, I'm not sure it did).

After all this, I carried on searching, unhappy with still needing to boot into 2 OSs every time I wanted to use Windows with a mouse. I came across this blog post online: https://forums.macrumors.com/threads/bootcamp-windows-7-magic-mouse-wont-work.1519990/. As a result, I booted into macOS, unpaired my Magic Mouse, booted back into Windows and paired it from there. And it worked! All was well in the world for a few weeks. 

However, I then came to my computer one day and the mouse wasn't responding again, again in spite of all the usual tricks. Getting desperate, I looked around and read somewhere (sorry the link eludes me) that uninstalling the driver and restarting would cause it to reinstall and could fix some common ailments. So I tried it. Restarted, and the mouse was still not working. I then decided to unpair and try re-pairing with the computer. However, when I tried this, I kept receiving an error saying that the Bluetooth device cannot authenticate (cannot remember the exact phraseology). I tried for days but couldn't resolve this, so eventually I resorted to booting into macOS, disconnecting and reconnecting the mouse, then booting into Windows, and voila, I can at least move and left click the mouse again. 

however, now I can't scroll or right click the mouse, which is a royal pain!

Anyone know how I can my mouse properly working between Windows 10 and macOS? Do I have to just buy a separate mouse? Nuke my Windows partition and start again? Or is there a way of having this pair work nicely?

Any help really, really appreciated. This has been a pain for months.
Finally, I seem to have solved this!

@BernardWei kind of lead me in the right general direction by suggesting I download the drivers from Apple again. Well, "download" is putting it strongly. I naturally went here to get them (which is wrong): https://support.apple.com/downloads/bootcamp

The downloads I was seeing on the Apple support site are Bootcamp v5.x versions. Which, according to a help article I stumbled across, aren't for Win 10, but are for earlier Windows versions (7 and the likes), which probably explains my issues when trying to use it. Please correct me if I'm wrong.

I decided to give downloading the support software via the Bootcamp Assistant in macOS another go. I originally got the idea from here: https://support.apple.com/en-us/HT204923

Previous attempts had seen the software not download and give a rather ambiguous error. Turns out that I had skipped over the bit about it needing to be in a FAT32 format. I was trying to save the data onto an external harddrive which I'm guessing isn't (I haven't got it plugged in to check right now). I then tried using the only USB drive I had, which only had 1GB of space, and the Bootcamp Assistant then failed for a lack of space reason, understandably. Having family to blag a larger USB drive from is useful if you come into this situation.

Here's the steps I took, once I had a large enough FAT32 USB drive.


Boot into macOS
Open up the Bootcamp Assistant.
Go into the actions menu and select Download Windows Support Software.
Select your ready-to-go USB drive as the destination and start the download.
You may be prompted for a password. Double check what it's asking and enter it if you're comfortable.
Go and get a hot drink, this will take a little while.
Once done, reboot into Windows.
Navigate to your USB drive. Go to WindowsSupport/Bootcamp/Setup.exe and run that. 
Pick repair.
Find something to do for a few minutes while the drivers on your machine are repaired. You may, like me, see multiple balloons pop up telling you that Windows is getting each one of your Bluetooth devices ready, and then again when they're done. You may also see a console Window running a disk partitioning tool. Don't worry, I think it just sets the default start up disk. I could be wrong, but nothing seems a miss yet.
Once done, you'll be prompted to restart. Do so. Your computer may try to boot to macOS, so use the options key to choose Windows as normal.
When your Windows machine restarts, your Bluetooth devices should behave themselves as expected.


Voila!

It's been a long time getting here, but I'm hopeful my mouse shinanigans are over for the time being. I'll reopen this question if my mouse stops connecting again as before. But for now, this is resolved!

EDIT: 1 day later, mouse isn't connecting again, and requires a restart into macOS to get it going. Which is back where we were at the beginning. That's fine, at least I know after a couple of restarts I'll have a fully working mouse again. 

Another side effect of doing this was that my external harddrive that normally runs as the D drive got de-lettered, as it were. I did a small amount of panicking before coming acoss this guide which helped me discover that all my previous data wasn't gone, but just needed re-assigning a drive letter from diskmgmt.msc: https://www.makeuseof.com/tag/external-drive-not-recognized-this-is-how-to-fix-it-in-windows/