iATKOS | United Kingdom

Goal: Erase your internal disk (not the “macOS Base System”) and install macOS (Big Sur or similar) safely.

Important: This will delete everything on your iMac. Make sure you have a backup first.

What You Are Seeing

• macOS Base System / Container disk5 – this is the temporary system from the installer. Do NOT erase this.
• Fusion Drive / Macintosh HD (under “Internal”) – this is your real internal disk. This is the one you erase and install macOS on.
• Generic – SD/MMC Media / Install macOS Big Sur – your external installer drive (USB/SD). Don’t erase this either.

Step 1 – Boot from the macOS Installer

1. Connect your Install macOS Big Sur USB/SD drive.
2. Turn on the iMac and immediately hold Option (⌥).
3. Choose the installer volume (e.g. Install macOS Big Sur) and press Enter.
4. When the installer loads, choose your language if asked.

Step 2 – Open Disk Utility and Show All Devices

1. In the macOS Utilities window, click Disk Utility and then Continue.
2. At the top-left of Disk Utility, click View > Show All Devices.
3. Now you should see a tree like:
   • Internal → Fusion Drive → Macintosh HD
   • External → your installer drive
   • Disk Images → Apple disk image Media → Container disk5 → macOS Base System

Step 3 – Erase the Correct Disk

1. Under Internal, click the very top item for your internal drive, e.g. Fusion Drive (not the volume “Macintosh HD”, and not “macOS Base System”).
2. Click the Erase button in the toolbar.
3. Use these settings:
   • Name: Macintosh HD (or whatever you like)
   • Format: APFS
   • Scheme: GUID Partition Map
4. Click Erase and wait until the process finishes.
5. When done, you should see the new volume (for example, Macintosh HD) under the internal drive.
6. Close Disk Utility (red button in the top-left).

Step 4 – Install macOS on the New Volume

1. Back in the macOS Utilities window, choose Install macOS and click Continue.
2. Follow the prompts until it asks where to install macOS.
3. Select the internal volume you just erased (for example, Macintosh HD), not “macOS Base System” and not the external installer drive.
4. Click Install. The iMac will restart several times; let it finish.

Step 5 – Initial Setup

1. After installation, your iMac will boot into the setup assistant.
2. Choose your country, Wi-Fi, Apple ID, etc., following the on-screen steps.
3. When you reach the desktop, the new system is ready.

Quick Recap

• Target to erase: internal Fusion Drive (top device under Internal).
• Do NOT erase: macOS Base System or Container disk5 (these belong to the installer).
• Install macOS onto: the newly erased internal volume (e.g., Macintosh HD).

OCLP and fusion drive on 2014 Imac... Hi Guys, sorry for the asshat first post, but I'm having a crisis of confidence trying to update the OS on my recently acquired Imac. It's a 2014 that I bought thinking it was a 2020 (my mistake!), but I'm struggling to know which drive to erase and where to install the boot file. Other tutorials I've watched (StaemPunk for one) specify a 'container', but all I can see at the top of the drive menu is as pictured.. I'm reluctant to start erasing and installing the boot file without knowing exactly what and which one .. Any help identifying which of the directories shown is the one to use..? Cheers - Greg submitted by /u/Greggynormous [link] [comments]

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