Adobe Illustrator saving files as .def

Hi @Mario D 

Apologies for this experience. As @Mylenium mentioned above, we need a few more details to assist you further. 

• OS and version of Illustrator
• Is the issue related to a specific document? If yes, could you please share the document with us? You can upload it to the Creative Cloud (https://helpx.adobe.com/in/creative-cloud/help/share.html) and share the download link.
• Where are you saving the files (Network Drive/ System HDD/ Shared Folder on System/ External HDD)?
• Has it started happening after the recent update?

We're here to help, just need more info.

Regards,

Srishti

Without any system info and critical details like save locations, contents  and file sitzes of the AI files and so on nobody can tell you anything. This simply sounds like you have issues with your storage devices.

Mylenium