There are many examples in which SketchUp files, especially larger ones, have been fatally corrupted when they've been saved directly to an external drive location. In many cases the data winds up being replaced with zeroes. Often all of the data is replaced with zeroes.
The general adice is to work on the file while it is saved directly to an internal drive. If you need the file to be on a server drive or out in the cloud, copy it to that location after a successful save to the internal drive.
Also, you should be regularly purging unused content from your models, keeping material file sizes reasonable, and keeping polygon counts under control.
