Tried to factory reset, now have a Mac harddrive and a new one called "Update" and can't run disk utility from recovery

To perform a clean install of macOS you need to boot the laptop to Recovery Mode (Command + R), Internet Recovery Mode (Command + Option + R), or from a bootable macOS USB installer. Once booted to the macOS installer launch Disk Utility and erase the physical drive as GUID partition and MacOS Extended (Journaled), or as GUID partition and APFS (top option) if the latter is available as an option. You may need to click "View" within Disk Utility and select "Show All Devices" so the physical drive appears on the left pane of Disk Utility. After successfully erasing the whole drive, quit Disk Utility and select "Reinstall macOS". This is only for Intel Macs.

If you have a 2018+ T2 Mac or M1 Mac, then you probably should "Restore" the firmware which requires access to another Mac running macOS 10.15+ as this will perform a clean install of macOS and also clear out & refresh the T2 security settings.

Revive or restore an Intel-based Mac using Apple Configurator 2 - Apple Support

Revive or restore a Mac with Apple silicon using Apple Configurator 2 - Apple Support

FYI, here is what the previous owner should have done prior to selling the Mac:

What to do before you sell, give away, or trade in your Mac - Apple Support

In the future when asking for assistance on these forums, please provide the exact model of your Mac since each model has their own unique issues and resolutions.

The serial number is on the case bottom in quite small faint font.

I understand the need to reset, but in cases like this with a properly operating Mac, I set up my own admin user, and completely remove the former owner's user. Of course you have to be able to login with the former user's admin password which isn't always available.

Previous owner influence is totally gone and I can then reset iCloud Drive, Apple Account and etc. to my specs.

ku4hx wrote:

The serial number is on the case bottom in quite small faint font.

I understand the need to reset, but in cases like this with a properly operating Mac, I set up my own admin user, and completely remove the former owner's user. Of course you have to be able to login with the former user's admin password which isn't always available.

Previous owner influence is totally gone and I can then reset iCloud Drive, Apple Account and etc. to my specs.

I'm sorry, but this is not correct. Just deleting the original user account may still leave behind all sorts of software issues with the computer and possibly software that is not licensed for another user to use. Who knows what nasty surprises await the user (malware/adware, unnecessary third party drivers, or even some unknown system modifications/configurations, etc.). Users should ALWAYS perform a clean install of the OS on any pre-owned computer.

For a Mac this is even more critical because performing a clean install can reveal whether the Mac has a firmware password enabled, or whether the laptop is still being managed by a previous owner. If either of these conditions exist, the person should return the laptop for their money back.