I have at present been able to mount and unmount firewire drive but not sure how i increment the mount number
Your script needs to count how many mounted drives there are first and then +1 using expr/bc. Here is an example of what I mean. In my test directory I have created a.txt and b.txt, so there are 2 .txt files. I'm using expr to add 1 and outputting what the result would be:
count=`ls *.txt | wc -l`
number=`expr $count + 1`
echo "Count: $count"
echo "Incrementing that would be: $number"