Why is Convert to APFS Failing? I have an SD card formatted as Mac OS Extended that was imaged with the Apple Diagnostics disk image. Holding option to boot to this SD card on a Mac works as expected.

I tried to use Disk Utility to convert the only volume on the SD card to APFS, and Disk Utility presents the error


  An internal error has occurred. Operation failed…


Is there an alternate way to make this work or otherwise a way to figure out exactly what's going wrong with the APFS conversion? Thanks!

Just as an update, with a Mac OS Extended USB drive connected, this is the output of diskutil list.

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         250.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            240.1 GB   disk1s1
   2:                APFS Volume Preboot                 19.7 MB    disk1s2
   3:                APFS Volume Recovery                509.8 MB   disk1s3
   4:                APFS Volume VM                      1.1 GB     disk1s4

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *15.8 GB    disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                  Apple_HFS AppleDiagnostics        15.5 GB    disk3s
The only macOS version that can boot from a APFS container is High Sierra. In your case, the Apple_HFS volume does not contain not High Sierra. There is no way to convert to APFS and remain bootable.