I did my whole return without saving and then got TT401 trying to save. None of the suggested options helped (saving to a different file name, saving to a different drive, etc). I have lots of drive space and nothing is write protected.
I started my return over from scratch and saved after every screen where I entered information and never encountered the error again.
This is an obvious software bug that Intuit should fix, and make their support people aware of so that they don't treat people calling for help with it as if they are stupid
Please try saving the file to a different location on your hard drive or with a different name. You can do this by choosing File> Save As from the top left corner.
You can also try renaming the .ttbk file to .tt18 – the .ttbk is the backup file, and it may have save the changes you made to your file.
Also, if you installed the software from the CD, to prevent further issues, you should uninstall it and then use the download from our website to reinstall the software. You can download TurboTax from here: https://turbotax.community.intuit.ca/replies/3001486