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There are several methods to check for a file or folder's existence. However, the fastest and most reliable is attempting to aliasize its path. Let's go to the sample (with a file, but it's the same for a folder or disk):
set unknownFile to "path:to:this stuff.html"
--> file exists, go ahead
on error --> file doesn't exist
--> do whatever
The trick is that an alias is a file or folder which exists. So, if file "this stuff.html" does not exist in folder "to" in disk "path", you will receive an error.