Downgrade apfs to hfs+ in high sierra using Terminal

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I'm a total newbie re Terminal, but need to downgrade the SSD on my iMac from apfs to hfs+. I tried disk utility but it didn't work (won't allow erasing once apfs is in place). So I went to Terminal, typed in 'diskutil list' and this is what came up:
/dev/disk0 (internal, physical):

/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *121.3 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_APFS Container disk2 121.1 GB disk0s2

/dev/disk2 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +121.1 GB disk2
Physical Store disk0s2
1: APFS Volume SSD DRIVE 13.3 GB disk2s1
2: APFS Volume Preboot 43.6 MB disk2s2
3: APFS Volume Recovery 1.0 GB disk2s3
4: APFS Volume VM 2.1 GB disk2s4


Not knowing which name was the correct one, the following is what I typed in based on one piece of advice that I read:
diskutil eraseDisk JHFS+ SSD DRIVE /dev/disk0s2

the response in Terminal was 'Unknown partition scheme: DRIVE'

Then I tried:
diskutil eraseDisk JHFS+ SSD DRIVE /dev/disk0
Again, Terminal replied in the same way.

'SSD DRIVE' is the name of the SSD on my desktop. Is that the correct name for the SSD?

What am I doing wrong?
 
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I'm a total newbie re Terminal, but need to downgrade the SSD on my iMac from apfs to hfs+. I tried disk utility but it didn't work (won't allow erasing once apfs is in place). So I went to Terminal, typed in 'diskutil list' and this is what came up:
/dev/disk0 (internal, physical):

/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *121.3 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_APFS Container disk2 121.1 GB disk0s2

/dev/disk2 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +121.1 GB disk2
Physical Store disk0s2
1: APFS Volume SSD DRIVE 13.3 GB disk2s1
2: APFS Volume Preboot 43.6 MB disk2s2
3: APFS Volume Recovery 1.0 GB disk2s3
4: APFS Volume VM 2.1 GB disk2s4


Not knowing which name was the correct one, the following is what I typed in based on one piece of advice that I read:
diskutil eraseDisk JHFS+ SSD DRIVE /dev/disk0s2

the response in Terminal was 'Unknown partition scheme: DRIVE'

Then I tried:
diskutil eraseDisk JHFS+ SSD DRIVE /dev/disk0
Again, Terminal replied in the same way.

'SSD DRIVE' is the name of the SSD on my desktop. Is that the correct name for the SSD?

What am I doing wrong?
You cannot reformat the drive you are running from. And I don’t know of a way to change the format of any volume, in this case APFS to HFS+, without destroying all information. If you’re referring to another SSD other than your boot drive, then you should be able to reformat it to HFS+ using Disk Utility. Just select the drive itself (the top category, the “device,” for the drive) and then click on Erase.

(Make sure that the View pulldown has “Show All Devices” selected if you do not see the device on the sidebar.)
 
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Thanks for responding. So, if I had a bootable external, I should be able to use Disk Utility or Terminal from the external to reformat the internal SSD?
 
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Thanks for responding. So, if I had a bootable external, I should be able to use Disk Utility or Terminal from the external to reformat the internal SSD?
As long as you are able to boot from the external, any other connected drive, including the Mac’s original, can be erased and reformatted. I probably don’t understand why you need to reformat the internal to HFS+, but the newer versions of macOS might not be able run properly unless it’s from APFS. However, as long as you have proven the fact that the Mac will function from an attached volume, you should be able to reformat the internal, as well as reformat back to APFS to reinstall macOS.

Also, from Terminal, if you need to know the different variables to any terminal command, e.g. “APFS,” just type: “man apfs” and press return. Each variable should also have its own brief description.
 

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