Google Sitemap not working after loading over 57,000 products via easypopulate

I used to have less than 400 inventory items. Google Sitemap worked great. Then I used easypopulate to import over 57,000 parts, and now it won't work.

At Step 1 (STEP 1: Click [HERE] to create / update your site map), it opens up a new window, pauses on "Transferring data from MyStore.com". Then, after about 25 seconds, it times out, doesn't give me any error...nothing. It just reads:

Google Sitemap Generation (v 1.3.19 20.04.2008 17:24) started

Save path - "/home/MyStore/public_html/"
Base URL - "http://MyStore.com/"
File "robots.txt" not found in save path - "/home/MyStore/public_html/robots.txt"

Anybody else experience the same problem, when working with lots of products?

I re-posted this question in the Google SitemapAdmin thread.