I've been doing the following for a couple of years, I do it several times every day, on most days of the week. Prior to support for .DOCX in calibre core, I used the DOCX Input PI, prior to that I used .DOC.
Where 'whatever' is a calibre book list entry
- convert 'whatever'.xxx (PRC, MOBI, TXT etc) to 'whatever'.RTF
- open 'whatever'.RTF in Word via View - press 'v', select RTF
- save 'whatever'.RTF as 'whatever'.DOCX in Word.
Save is in the same folder as the RTF (ie the calibre library\author\book (nn) folder) with exactly the same file name as the RTF. That is the default behaviour for Word's Save As feature - ie same location, same file name with another extension.
It is important that the file name is the same, if I were to change it I would end up with an extra .DOCX in the book folder and Library Check would subsequently complain
- close 'whatever'.DOCX in Word - if I don't the file will remain locked
- from calibre, open the book folder for the 'whatever' book in Windows Explorer (WE) - press 'o'
- drag the 'whatever'.DOCX file from the WE window to the 'whatever' Book Details panel and drop it - if the file is locked calibre will pop an error message
- close the WE window
- calibre now has a registered DOCX format for the 'whatever' book
- open 'whatever'.DOCX in Word via View to edit the contents
I never vary my process, I execute the steps contiguously and swiftly. I've never needed to restore a library from backup, I never get any errors from check library etc etc.
I can't see any reason why this process would work for me with RTF and DOCX files in Microsoft Word (Windows and NTFS), and not for someone else using say TXT and ODT files in OOo Writer. If it doesn't then its a defect or a deficiency in Writer, the OS or file system being used - which would surprise me.
BR