Symptom
The File Sharing computer unexpectedly goes to sleep while it appears to be actively File Sharing. You receive this message:
"The file server's connection has unexpectedly closed down [time and date]."
Solution
To prevent this behavior, follow these steps:
1. Open System Preferences.
2. Click the Energy Saver icon.
3. Click the Sleep tab.
4. Set the "Put system to sleep whenever it is inactive for" slider to Never.
5. Quit System Preferences.
This is the only certain way to prevent the computer from going to sleep while it is sharing files. Being connected to a sharing computer (having it mounted) is not sufficient in itself to prevent the computer from going to sleep or to reset the sleep clock. You might also encounter this behavior if the software you are using locally caches the file being read from the sharing computer. Though it might appear that you are actively reading the file from the sharing computer, there may be no activity between the two at the moment that the defined sleep period expires.