The most common causes of a truncated file are that the save is interrupted somehow (e.g. shutting off the computer, shutting down Surfer, or an error was generated during saving) or that the file was saved to a corrupted block on the disk.

Check your backups (if applicable) to see if you have a backup copy that contains the full file. You may also want to consider turning on Surfer's backup feature: 

1. Click File | Options.
2. In the Options dialog, click on General.
3. On the right side of the dialog, check the box in the Backup files before saving field.
   

Enabling that option will create a backup file in case the original becomes corrupted in any way. That may help in the future, but not for the current file.

Updated November, 2021