creating APFS fusion drive / conversion to APFS from HFS+ fusion drive fails - "you are trying to create an upside down Fusion" I have 1TB Internal SSD and 512GB SD card in a SD slot.

I can create a fusion drive in HFS+ format, and the drive was running fine on my previous El Capitan system. Now that I upgraded my system to Catalina, I wanted to format this fusion drive volume in APFS (currently running Catalina from external hard disk drive, not this fusion drive). But the conversion always fails with either of these 2 error messages:

1:----------------------------------
A problem occurred during conversion from HFS to APFS
APFS Converter exit code is 1
Aborted conversion in commit mode so will restore type
Setting type of disk0s2 to CS
Changing the physical disk partition type in shared mode
Partition modification attempt count was 1
Setting type of disk4s2 to CS
Changing the physical disk partition type in shared mode
Partition modification attempt count was 1
Not mounting APFS Volume
Exiting conversion operations with error code -69860
Error: -69860: The underlying task reported failure on exit


and

2:---------------------------------
It looks like you are trying to create an upside down Fusion:
Main tier (988 GB) is larger than Tier2 (462 GB)




I have no clue about the error message 1, but assuming from the error message 2, APFS fusion drive can not be created if the main drive ( SSD ) is larger than the secondary one? If so, is there a way to force it to make the fusion drive anyway?

what I did to convert:


diskutil apfs convert disk1
diskutil apfs createContainer disk1 disk2
from Disk Utility's Edit menu: "Convert to APFS" ( unmounted the target volume first too )
used a Carbon Copy Cloner and cloned Catalina's bootable volume on external hard disk to the target fusion drive
from OSX Catalina Install Disc


I'm really stuck now. Is there anything else I can try? I appreciate your help, thanks...!
I finally took the plunge and upgraded my iMac to Catalina.  It originally had the fusion drive with SSD and spinning hard drive.  However, I upgraded the HDD to a Samsung SSD and combined it with the original SSD to create one large CoreStorage logical volume (can’t call it “Fusion” anymore.”)

While trying to convert this logical volume, I ran into all of the same issues you did, so I was able to replicate the problem.  However, after about an hour of throwing to force things, I found the trick: reverse the order of the drives

What you think it should be:

$ diskutil APFS create disk0 disk1 “MyVolume”


Reverse the disk identifiers to make it work

$ diskutil APFS create disk1 disk0 “MyVolume”


(I’ll post all of the command output shortly...I’m posting from my iPad as Catalina installs onto a APFS “Fusion” drive).