Missing file problems and other odd behavior may occur when working from an external drive.
The path to an external drive is subject to change, which may cause the loss of Backup and Auto-Save files.
Best Practice to Avoid Problems
Never save your original file to an external drive.
Never work on an original file loaded from an external drive.
Always open files from your local hard disc and save your work to the local hard disc.
Afterward, copy your original file from your local hard disc to other drives if you wish.
• Auto-Save is enabled within the General panel of the Preferences dialog box.
An Auto-Save file is a temporary file available on the file system should SketchUp abnormally exit, aka crash.
The temporary file’s name will be a combination of the word “AutoSave” with the original filename, such as AutoSave_hotel.skp.
When an abnormal exit occurs, the Auto-Save file is saved in the directory where the original .skp file exists.
• Create Backup is enabled within the General panel of the Preferences dialog box.
When you save a SketchUp (.skp) file and Create Backup is enabled, the existing file will be converted to a backup file (.skb), and the new drawing is saved in place of that currently existing file (.skp). Thus, the backup (.skb) file is one save behind the (.skp) model file.
The Backup file (.skb) is saved in the directory where the original .skp file exists.
File naming varies slightly between PC and Mac … See these Help Articles
Does SketchUp create auto-save files? — SketchUp Help
Backup Files — SketchUp Help