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#1 2021-02-21 13:32:01

waliworld
Member
Registered: 2015-12-16
Posts: 11

Reinstalling windows within existing triple boot?

I got a mid-2015 MBP retina (A1398) with a triple boot of OSX 10.11.5, windows server 2019 and finally arch using systemd-boot for the loader. Windows server was an inane mistake to install. My question is about the windows 10 installer: if I select the "advanced install", format the windows server partition in place and point the installer to that partition... Will the installer leave the efi partition alone? I'm hoping I can just deal with the windows partition here but I don't trust the windows installer and I couldn't find any particular answer to this in my bit of Googling before I came here. So does the installer play nice? Thanks!

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#2 2021-02-21 14:04:35

V1del
Forum Moderator
Registered: 2012-10-16
Posts: 12,196

Re: Reinstalling windows within existing triple boot?

You need the Windows installer to use the ESP so that it can place it's own EFI files there, but it should only add it's own files and not remove existing ones.

In any case you'll want to ask this kind of question on a Windows support forum.

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#3 2021-02-21 15:08:47

frostschutz
Member
Registered: 2013-11-15
Posts: 1,031

Re: Reinstalling windows within existing triple boot?

My question is about the windows 10 installer: if I select the "advanced install", format the windows server partition in place and point the installer to that partition... Will the installer leave the efi partition alone?

I don't know.

I didn't trust it, so I passed the first half of my SSD (loop device with offset and sizelimit to where I wanted the Windows install to be) to Qemu/KVM, installed Windows 10 virtually (without entering key), then rebooted it on bare metal. Worked fine for me, Windows 10 had no issue with the platform change VM to Hardware (provided you enter your key only when hardware configuration is final, as otherwise I assume it would look like using the same key on different PCs).

Using KVM to install Windows 10 was a lot more comfortable than unplugging drives, which I used to do for Windows 7 and earlier...

But you should be familiar with Qemu/KVM already

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#4 2021-02-21 19:23:53

waliworld
Member
Registered: 2015-12-16
Posts: 11

Re: Reinstalling windows within existing triple boot?

frostschutz wrote:

I didn't trust it, so I passed the first half of my SSD (loop device with offset and sizelimit to where I wanted the Windows install to be) to Qemu/KVM, installed Windows 10 virtually (without entering key), then rebooted it on bare metal.

Ok this sounds like a pretty clever workaround. I'm not familiar enough with qemu setups but I'll figure that out. I'm curious though what did the installer see of your drive within qemu? An empty disk or the old partition? If it's the old partition I'm assuming you formatted and installed to that existing partition in which case that's exactly what I'm going to do! Thank you!

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#5 2021-02-21 19:35:24

frostschutz
Member
Registered: 2013-11-15
Posts: 1,031

Re: Reinstalling windows within existing triple boot?

I'm curious though what did the installer see of your drive within qemu?

It sees a virtual drive, it partitions that drive, installs to that drive, makes it bootable, etc. I just re-added my untouched Linux partition afterwards. My bootloader is separate anyways (I boot off USB stick) so I could leave whatever the Windows installer sets up untouched.

Basically the exact steps depends on your situation and preferences. It helps if you are familiar with partition tables and offsets and everything else really.

I'm not sure if anyone else uses this method, if so there may be a more detailed guide somewhere.

Windows installer aside, even Windows itself might touch your drives once you boot it. If you have any unpartitioned disks, it might be dangerous as Windows might create partition tables unasked.

If any of your partitions are not flagged to be Linux partition types, Windows might try to format them.

Have a backup ;-)

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#6 2021-02-22 04:23:03

kermit63
Member
Registered: 2018-07-04
Posts: 69

Re: Reinstalling windows within existing triple boot?

Not sure if this is still applicable since I no longer dual-boot Windows 10 past 3+ years.

Windows 10 shenanigans goes well beyond the installation phase. Windows has this nasty habit of creating new partitions on-the-fly during major system updates, and then renumbering the partitions that falls below the newly-created partition. For example:

/dev/sda1 EFI partition
/dev/sda2 Windows System Reserve
/dev/sda3 Windoes partition
/dev/sda4, /dev/sdax ... linux partitions

During an update Windows may decide to resize /dev/sda3 and create a new partition which it will renumber to /dev/sda4. As a result, the previous /dev/sda4 will now become /dev/sda5 while subsequent partitions will become /dev/sdax+1.

The GRUB bootloader is oblivious to the change if you use UUID to identity partitions, but would still need to recreate the EFI boot entry to point to the correct partition. What I used to do before was to place my arch partitions before the Windows partitions so they're not affected by Windows' partition renumbering shenanigan.

I don't know how this impacts systemd-boot since I don't actively use it. Experimented with it (systemd-boot) before, got it to work, but I still use GRUB since I need its ISO booting capabilities that systemd-boot doesn't have.

I have since moved Win 10 to a VM inside my arch install, so I'm not sure if this is still a problem with Windows 10 updates.

Last edited by kermit63 (2021-02-22 06:51:31)

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