When you connect your N900 to a computer via USB and select Mass Storage Mode, your N900 will try to close any files it has been using on the internal 32 GB storage and any micro SD/SDHC card you have installed. It must do so before it can turn those storage volumes over to Windows. Maemo and Windows can't both use storage on your N900 at the same time.

The particular message of which you've written above indicates the N900 has failed to close all files, so it can't give Windows access to the cards. Therefore, the N900 has stayed in Charging mode rather than switching to Mass Storage.

Some application on your N900 is keeping files open and blocking the switch to Mass Storage. Look at what's running on your N900, and close applications that might be using files on the 32 GB storage or the micro SD/SDHC card. Then, try reconnecting to the computer and choosing Mass Storage again.