When you reply via email, the message does get posted on the forums. However it’s a fair question and there’s no issue discussing it in public!
Approximately 10 years ago the PC industry has switched away from MBR (“legacy BIOS”) boot, to UEFI. Endless supports dual boot with windows on both legacy MBR and UEFI, but I’m going to assume your system is using UEFI instead of MBR here. (Plus even in the MBR booting case we do not touch the MBR)
For UEFI boot, we install the Endless bootloader alongside the Windows bootloader in the EFI System Partition (ESP). The Windows bootloader is left there intact. Then we instruct the EFI firmware (which would previously have been referred to as the “BIOS”) to treat the Endless bootloader as the default boot option instead of Windows. And when booting, the Endless bootloader presents a menu which lets you select between Endless OS or Windows.
If something goes wrong on the boot/bootloader side, you can go back into your firmware setup menu (usually by pressing F2 or Del or similar right after turning the PC on) and reconfigure it to boot the Windows bootloader by default again.
That said, there are a couple of considerations when sharing the C: drive between Endless and Windows:
- We can’t install at all if Bitlocker disk encryption is enabled, in which case the installation app will immediately abort)
- We can’t boot Endless if Windows was last shutdown through the Hibernate option, in which case the Endless bootloader will refuse to boot Endless and ask you to first boot Windows then reboot.
We did face a couple of issues when we launched this clever C: drive sharing system 3 years ago - but none of those cases lead to Windows getting broken, only that Endless could not be booted.
Our present day experience indicates the biggest issues users face here are the disk encryption (so Endless install can’t happen at all), and standards-incompliant system firmware which ignores our request to activate the Endless bootloader (in which case the system will just boot straight into Windows).
In short, I don’t see any significant risk here. But at the same time, you hopefully have some decent backups, if nothing else because hard disks do occasionally die without warning.
Here is some more technical info on how this works and the problems we overcame.