I do the same but now i can't create a new Fusion drive. And can't even reformat Apple SSD ( 24Go) PCI Express in HFS....
I can only install OSX on the other drive. The PCI Express card is for moment lost in the Darkness of this New APFS systhem. I format cause OSX 10.13 can't be installed in APFS format grom my previous instalation.
It's only 24Go lost but PCI Express was a good point for fast runnig OSX....
I will try without this PCI Card for moment ...
I will check when Apple give us a solution to resolve this problem and when we can agin created a FD with Terminal between this 2 drive in APFS
It is possible yes, I managed to format my fusion drive to APFS as follows via the command line:
diskutil apfs createContainer disk0 disk1
diskutil apfs addVolume disk3 APFS “Macintosh HD”
Once you complete the second command the 2 drives are replaced with the Fusion Drive when viewed in Disk Utility.
Your disk references might be different depending on you config.
I am going to try this.
I reformatted my MBP fine APFS and installed 10.13. So I thought I would give the imac a try…
My first attempt split my iMac late 2014 1TB fusion drive. I reconnected it in terminal and reformatted as journaled and reinstalled 10.13.
But the iMac seems slugish and takes longer to reboot. I was wondering if the OS got installed onto the SSD part of the drive? Is there a way to check? Or to manuall select the SSD as installation of the OS?
Then I will try your command line and try to reformat to APFS