wM FS1 has the following steps:

Step1: receive input from system1.
Step2: Call system2 for data.
System 2 retrieves data and places it on mounted drive.
Step3: Should retrieve the data from mounted drive
Step4. send the response back to system 1.

How can Step 3 be achieved:
If I set up the file polling for the data on the mounted drive,
I have the option of setting up: “Processing Service”. How can I mention “FS1 : Step3” as processing service.

If I create another FS2 to retrieve the data from Mounted drive, how can I pass this data to
“Step3” of FS1 to respond the system 1 with the data.

Or is there any other way to do this other than File polling?

If I understand correctly, you’re looking for a service to retrieve a file from the disk, right? Then take a look at WmPublic/pub.file:getFile.