1) In case u get an “unknown error” and you cant save the file, try putting off all the layers and then saving the file. That way you will save time on “Generating full resolution composite” which takes a very long time (especially for big files) and directly jump to saving the psd file. This had come in extremely handy for me
when I have to save a file before leaving.
2) If that layer off thing doesn’t work out you can also try saving the same file in a Tif, Tiff format.
3) If you want to reduce the file size of your psd you can try using the inbuilt fill colour and mask things out instead of making a new layer and then drawing on it.

The above techniques I have tried out personally and things have worked out, as we work with heavy files at times. Secondly if your file size goes beyond 2 gb you cant save a psd file.