Word 2007 has a habit of suddenly deciding all by itself that my document is read only and not letting me save it.

I am working from a memory stick, which might make some difference.

The problem is called Microsoft.

is there a switch on the side of the memory stick?
there often is, a Read only/read write toggle.

can you "save as" and give it a new name, then it should save as editable?

Have you opened the document on another pc without 2007? That is when it happens to me and I need to re save the document.

It's not a permanent problem – all I have to do is save it under a new name, then delete the original and rename the new one.

Nothing to do with disc write protection.

Saved in docx format, and only been opened on this computer.

I'll be able to save for hours, until all of a sudden once I'll hit CTRL-S and it thinks it's read only. But carries on fine with a new name.

excel 2007 does this too