Partition Mojave HD to run Sierra

Started 3 months ago | Discussions thread
silentstorm Senior Member • Posts: 1,466
My shot "in the dark"

After reading through the replies, here's my take.

If you already have a newer OS (Mojave) in the laptop, you can't install older OS versions, not even external.

If you are really desperate to get old OS back, bite the bullet. Make sure backup all data!

1) Put in the installer USB of Mojave and boot into this installer during restart/bootup.

2) Erase disk using the Disk Utility. Do disk partitioning from it, create 2 partitions capacity of your choice.

3) Mojave has an option to format in HFS+ or APFS. Choose HFS+ for 1st partition.

4) Exit installer and shut down.

5) Plug in Sierra installer USB and boot into it.

6) Follow through the installation guide and select the 1st HFS+ drive to install Sierra.

7) Installation completed. Shutdown. Plug in Mojave installer USB.

8) Bootup into this Mojave USB installer disk.

9) Follow the installation process until you need to select which drive to install into.

10) Choose the 2nd partition and format it in APFS. Then proceed on.

Now I don't know if using time machine is a good idea to clone onto the 2nd partition. But I HIGHLY RECOMMEND that you fresh/clean install everything into Mojave to avoid any possible corruption on the boot sector of the drive.

Good luck!

EDIT:

Like to add.  If after erasing & creating new partitions, the Sierra OS still prompts you the error of an older OS, this means the drive is not thoroughly cleaned/wiped.  You have to find a way to thoroughly wipe the drive in this case.

Remove the drive from the laptop. Put it in an ext enclosure. Borrow from a friend or neighbour Windows computer. Plug in and use DOS command to do a clean wipe via Diskpart.exe utility.

Yes.  This can be a hassle, but if you need it, just do it.

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