<p>How can I poll disk activity in Applescript? Check to see if disk X is being read, written, or idle every N seconds and do something.</p>

In general, polling is less efficient than being notified when something happens. Additionally, if you're checking whether something is reading from a disk, you will probably be accessing said disk yourself, possibly influencing what you're trying to observe.
Since 10.5, OSX includes something called the File System Events framework, which provides course-grained notifications of changes to the file system. The problem in your case is that this is Objective-C only. Apple has some nice
documentation
about this API.
Fortunately, there is also the
```
call method
```
AppleScript command. This allows you to work with Objective-C objects from within AppleScript. Here's the
documentation
on that.
I have no experience with either, hence the documentation references. Hopefully, this should get you going.