You won't see the storage area while you're connected in mass storage mode. When you connect in mass storage mode, it makes the devices available at a low level to the PC.
This means they can't also be available on the N900, so get unmounted.If you're not in mass storage mode, you should be able to see the partition mounted on the N900, otherwise you should be able to see it as a drive on the PC. If you can see it okay, and it shows enough space, then it must be a bug in Map Loader.I've seen some apps which appear to be using too small variables for checking available space - if you have over 16G free then it discounts the first 16G.
If this is the case, you could try putting data on the drive to take it below 16G free, then see whether this makes a difference to Map Loader. OK, so it works fine when using the Nokia Map Loader in XP. Make sure its in Mass Storage Mode, and then download the maps you want (deleting original map data from device).When you open up ovi maps, it WILL download data but I think thats related to AGPS because you can VERY QUICKLY scroll across the land mass (in my case the entire UK) and it will already be rendered (even if you zoom right the way in). I'm thinking that even with a fast wi-fi network, you would notice a slight delay before the data was downloaded and rendered but in this case there is no delay and the maps are rendered fine.