You may have named ranges that refer to other sheets. One of my
colleagues sent me a file that had been passed on from person to
person to person and it had about 1500 named ranges, all pointing to
other files.

If you've checked all the possible sources of external links and
simply want to disable to startup prompt, on the menu click Edit
Links and select the Startup Prompt button in the lower left corner.
Choose the radio button that you prefer.

Dave O
Eschew obfuscation