Add a volume to /etc/fstab to prevent auto-mounting. See http://akrabat.com/prevent-an-external-drive-from-auto-mounting-on-macos/

no_automount

#!/usr/bin/env bash

# Usage: ./no_automount My\ Disk
NAME=$1
if [ -z "$NAME" ] ; then
    echo "Usage: no_automount {Disk Name}"
    exit 1
fi

FSTAB=/etc/fstab

# Add an volume as not auto-mounted to the /etc/fstab file
# by it's identifier. Also pass in the volume name to add a
# comment on that line so that we can identify it later. 
#
# example usage:
#       add_identifier disk4s1 "Swiftsure Clone"
#       add_identifier disk4s5 "Swiftsure Clone - Data"

function add_identifier {
    ID=$1
    VOLUME_NAME=$2
    if [ -z "$VOLUME_NAME" ] ; then
        echo "add_identifier() takes two parameters: ID and VOLUME_NAME"
        exit 2
    fi
    
    # get UUID and TYPE from `diskutil info $ID`
    UUID=`diskutil info "$ID" | grep "Volume UUID" | awk '{print $NF}'`
    TYPE=`diskutil info "$ID" | grep "Type (Bundle)" | awk '{print $NF}'`

    # Remove this UUID from fstab file
    sudo sed -i '' "/$UUID/d" $FSTAB

    # Add this UUID to fstab file
    echo "UUID=$UUID none $TYPE rw,noauto # $VOLUME_NAME" | sudo tee -a $FSTAB > /dev/null
    echo "Added $UUID ($VOLUME_NAME) to $FSTAB"
}

# Add all volumes that start with $NAME to the /etc/fstab such
# that they do not automout. 

# Get list of identifiers and volume names from `diskutil info`
LIST=`diskutil list | grep "$NAME"`

# Iterate over $LIST
echo "$LIST" | while read LINE
do 
    # Example of $LINE:
    #    1: APFS Volume Swiftsure Clone - Data 592.1 GB disk4s1

    # Extract disk identifier which is the last field on the line
    ID=`echo $LINE | awk '{print $NF}'`

    # Extract volume name in the middle of $LINE by: 
    #    1. remove all characters before $NAME using regex capture assign to $PARTIAL
    [[ ${LINE} =~ ($NAME.*) ]] && PARTIAL=${BASH_REMATCH[1]}
    #    2. Cut out the last 3 fields (size, units & ID) from $PARTIAL
    #       by reversing, cutting and un-reversing
    VOLUME_NAME=`echo $PARTIAL | rev | cut -d' ' -f 4- | rev`

    add_identifier $ID "$VOLUME_NAME"
done

# All done!
exit 0