On dual-boot macOS/Ubuntu Mojave install failed with "This disk cannot be used to startup your computer." My box is mid 2012 MacBook Pro, 1 TB HD, Sierra 10.12.6. It has dual-booted with Ubuntu using rEFInd until the High Sierra installation failed, after which I only used Ubuntu. After uninstalling rEFInd I'm attempting to upgrade the macOS side to Mojave and get the error above. I've already run First Aid on the Macintosh HD. Here's the partitioning:

# diskutil list disk0
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            500.4 GB   disk0s2
   3:                  Apple_HFS Recovery HD             650.0 MB   disk0s3
   4:           Linux Filesystem                         498.8 GB   disk0s4
# gpt -r show disk0
gpt show: disk0: Suspicious MBR at sector 0
       start        size  index  contents
           0           1         MBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640   977270656      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   977680296     1269536      3  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
   978949832   974313152      4  GPT part - 0FC63DAF-8483-4772-8E79-3D69D8477DE4
  1953262984      262151         
  1953525135          32         Sec GPT table
  1953525167           1         Sec GPT header


Is there a way to install Mojave without reformatting?
I deleted the Ubuntu partition and then used macOS utilities, Disk Utility and diskutil to recover that space. Then the upgrade to Mojave proceeded normally. At that point I used these utilities to resize the Macintosh HD partition to half the disk and created a following spacer partition of 128 MiB (see rEFInd forum). I then reinstalled rEFInd from macOS and Ubuntu in the remaining free space. The result is a working Mojave/Ubuntu dual-boot system.