How to Format exFAT on Mac with custom block size (Read only problem!) I'm trying to format a CF card on Mac Osx 10.14.4 to exFAT with a specific block size of 4MB, with this terminal command:

sudo newfs_exfat -b 4194304 -v exFAT /dev/disk3s1


It appears to have worked…

Volume name      : exFAT
Partition offset : 63 sectors (32256 bytes)
Volume size      : 125059009 sectors (64030212608 bytes)
Bytes per sector : 512
Bytes per cluster: 4194304
FAT offset       : 2048 sectors (1048576 bytes)
# FAT sectors    : 2048
Number of FATs   : 1
Cluster offset   : 4096 sectors (2097152 bytes)
# Clusters       : 15265
Volume Serial #  : 5e319fb9
Bitmap start     : 2
Bitmap file size : 1909
Upcase start     : 3
Upcase file size : 5836
Root start       : 4


…but then it refuses to mount the volume afterwards:

Volume on disk3s1 failed to mount
If the volume is damaged, try the "readOnly" option


Any ideas please?
Answering my own question here! 
Use this wonderful app, which mounts the disk perfectly:
exfat