set -e
sudo umount /drive || [ $? -eq 0 ]
echo do stuff

if sudo mount /drive; then
    : # exit status 0
else
    rc=$?
    [ $rc -ne 127 ] && exit $rc
fi

env status=0 bash -e -c '
    mycmd() { return $1; }
    if mycmd $status; then
        :
    else
        rc=$?
        [ $rc -ne 127 ] && exit $rc
    fi
    echo OK
'

sudo umount /drive

case $? in

        0)
                ;;
        127)
                ;;

        *)
                echo 'bailing!'
                exit 1
                ;;
esac

echo 'doing more stuff'