That is exactly what Peter was talking about.

The N900's eMMC (think of it as a kind of permanently soldered-in memory card) has three partitions:

1. 29 GB FAT, mounted as /home/MyDocs and usually just referred to as "MyDocs"
2. 2 GB etx2, mounted as /home and referred to either as "home" or as "opt", depending on who you ask and in what context
3. 768 MB swap

When you attach your N900 to a PC with a USB cable in the Mass Storage mode, the 29 GB MyDocs partition is unmounted (i.e., it becomes invisible to the phone) and is exposed in its entirety to the PC as an external USB storage.

Please note that the /home/MyDocs directory ("folder" in the Microsoft newspeak) still exists, only no longer as a mount point. Which means that if you try to read from or write to this directory, the phone will "see" it as part of the 2 GB "home" partition. The standard File Manager is aware of this and hides MyDocs in Mass Storage mode, but alternative file managers and of course the command line do no such tricks so any attempt to access /home/MyDocs goes to the "home" partition.

This situation persists for a few seconds, sometimes up to a minute, after unplugging the USB cable, until the phone re-mounts MyDocs again.

Once MyDocs is mounted, any content written to /home/MyDocs while it was seen as part of the "home" partition becomes shadowed by the mounted partition and effectively disappears from view, until you unmount MyDocs again. Thus, the contents of /home/MyDocs, as seen from the phone's point of view, changes depending on whether the phone is connected as Mass Storage or not. This is why the default File Manager hides MyDocs in the Mass Storage mode, to avoid confusing the user. But, as mentioned above, other file managers and CLI do no such thing.

I suggest using the command line to check the contents of /home/MyDocs while in the Mass Storage mode. It should be an empty directory but you may be surprised what you find. If you do find something there, move it somewhere else and, once the USB is unplugged and MyDocs mounted again, move it there. It would also free up space on the much smaller, 2 GB home partition.

I hope I did not make the above too confusing and/or too tedious. It is quite late and I am tired and prone to rambling incoherently.