When a model is created, the model file is saved with an .FDB extension. When you open an existing .FDB file, the program always immediately creates a backup of that file. The backup file has the same name as the .FDB file but it has an .fbk extension. The .fbk file is not updated unless you close the .FDB file and then reopen it.
The .fbk file is a binary file, not a text file. If your .FDB file should somehow become corrupted or lost, you always have a backup of the file in the .fbk file, as it was when you last opened the .FDB file. If you need to access the .fbk file, simply change its extension to .FDB and use it like a regular .FDB file.
Thus, in addition to saving your .FDB file frequently, it may also be advisable to close and reopen it from time to time to update the .fbk backup file.
Important Note: Use the Options menu > Auto Save Model command to generate reminders to save your model on a timely basis..