Automator and Apple script error handling

Hello guys,
I hope you can help me with this.
With Automator I made a task to start a program and after that run a applescript to mount my external hard drive. (don't know anything about scripting, found it on the web)
The task works perfectly when the external hdd can be mounted, but when it's already mounted the task won't close and I have to do a forced quit.
I think there must be something added to the script that it will quit or not run when the hdd is already mounted.

This is the script i'm using:

Can't help with that script but I might be able to fix the problem without using that script. Can you tell us a bit more about what the task accomplishes? I thought I had seen an action that solves this problem right away but it stops the workflow if the disk is not mounted