Issue with available space to reinstall MacOS Sierra on iMac

shabnamB wrote:

iMac computer belonged to a family member who passed away, I’m trying to sell it for his wife.

To sell, I’ve erased everything. Now I’m trying to reinstall MacOS Sierra but I don’t have enough space.

It requires 999.86 GB but I have 999.2 GB available (I think!)

I erased Internal disk using Format MacOS Extended (Journaled), Scheme GUID Partition Map.

You seem to be misunderstanding the info presented by the installer. That drive has 999.86 GB (nearly 1 TB) of free space available. There is nothing on it. Plenty of room to install an OS.

There is an Apple Disk Images that I tried to erase but cannot (not sure if required).

There is nothing you can do with the Apple Disk image. You can safely ignore it. It is irrelevant to the user in most cases.

Judging by the screenshots the iMac appears to be ready to accept installation of an OS.

Your late-2015 iMac can run macOS 12 Monterey. I suggest you download and install that. High Sierra is quite old and obsolete now.

Do you have another Mac available to you?

If so, please tell us exactly which model and year it is.

Using a relatively new model you may be able to follow the guidance for here to Create a bootable installer for macOS - Apple Support. You'll need a 16GB flash drive. Using a bootable flash drive you can install Monterey on the iMac.

Try booting into Internet Recovery Mode using Command + Option + R to attempt to access the online macOS installer. Theoretically this should access the online installer for the most recent version of macOS that is compatible with your Mac, however, some Macs may only access the online installer for the version of macOS which originally shipped with the computer from the factory regardless of the keys used for booting.

Otherwise you need access to another compatible Mac in order to create a bootable macOS USB installer. Generally you need:

• macOS 12.x -- model 2015 to mid-2022 (assuming a 2015 iMac)
• macOS 11.x -- model Late-2013 to mid-2021
• macOS 10.15 -- model 2012 to mid-2020 (Intel only)
• macOS 10.13 -- model Late-2009 to mid-2018
• macOS 10.11 -- model 2007 to 2015 (assuming a 2015 iMac)

You can confirm the other Mac is compatible with the OS by using the information in the following article since the model designations listed are just generalizations:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

These are your options for reinstalling macOS. I guess you can try erasing the "Macintosh HD" volume as well just in case there is something wrong with it. Normally I recommend erasing the whole physical drive, but currently you are likely booting into the local Recovery Mode to access the Sierra installer.

FYI, more than likely the internal Hard Drive is failing which is why you are getting this error. If the system was running very slow before being erased, then it makes this possibility even more likely. You can try running the Apple Diagnostics to see if any hardware issues are detected, but unfortunately the diagnostics rarely report any issues.

You can also try installing macOS to an external drive to see if that makes any difference. I would recommend booting into Internet Recovery Mode using Command + Option + R to bypass the potentially damaged local recovery mode located on the internal Hard Drive. If you can install & boot macOS from an external drive, then it will be possible to check the health of the internal Hard Drive.

I do not believe that is a 2015 model iMac. It would have had the same CPU but it ran at a higher speed than 3.1 GHz.

You really need to determine the exact model iMac. From the About This Mac screenshot you provided I believe it's closer to a 2011 model, either a mid-2011 or a late-2011 which may have shipped with installation media (DVDs — the former did, the latter didn't).

Do you have them, or can you find them? They will make installing an operating system much easier.

Use Identify your iMac model - Apple Support to determine its exact model. That information will lead to a more expedient solution.

I'm convinced that the OP's computer is an iMac (Retina 4k, 21.5-inch, Late 2015).

The combination of the CPU (i5 @ 3.1 GHz) and the GPU (Iris Pro 6200) would confirm that.

And it looks like HWTech has things pretty well covered as to getting an OS onto this Mac. 👍🏽

You're probably right. I was trying to find that specific combination CPU, clock speed, and GPU and gave up. That, and the fact it ought to have shipped with something earlier than Sierra confused me.

In that case it would not have had discs, and installing Monterey through Recovery would be the way to go.