OPERATING SYSTEM

..DjKaluji:

Perhaps to identify the build model year of iMac as best possible, by not using serial numbers;

can be a starting point. These startup, & erase/reinstall newer systems, or re-install to new HDD.

Online macOS disk utilities can be used in macOS recovery, to erase the Windows OS from macHD

and format it correctly to install the replacement macOS full system from the Apple servers archive.

• Identify your iMac model - Apple Support (shows Present 2021 -back to- Early 2009 models)

https://support.apple.com/en-us/HT201634

The 'internet recovery' idea has been around awhile.

• How to reinstall macOS - Apple Support

https://support.apple.com/en-us/HT204904

Note the information after these words:

"Other macOS installation options"

[Near bottom of same support article ~ see

link to downloadable installers* for macOS]

This article can re-install from scratch, to an erased storage drive, a new macOS; since these use

online utilities and downloadable installers. And this one article would suffice to do most that.

However should your build model iMac use both a small flash (SSD) and larger HDD for storage,

as some do.. That may show as 'two separate' hard drives; if the link between those was broken.

Certain models of iMac use that type of configuration. These things, you may need to know.

Following:

* Source of standalone 'downloadable installers' which may used on erased, or new HDD:

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

The MacOs should still be there. Have you tried to start it up to the boot menu that lets you change the OS to boot from?

Hold down the option key when you start up the Mac:

Mac startup key combinations - Apple Support

Welcome to the Apple Support Communities.

What year is your iMac? Do you have access to another Mac?

Jack