Why maps use more storage space than their actual file size
There’s a lot of confusion on the filesize of maps around. A lot of the map-files (cdt) are smaller than the usual “Allocation unit size” in a file system. With FAT16 that’s usually 32kbyte, with FAT32 and NTFS 4kbyte. So fitting a 500byte-file in the filesystem wastes lots of space, 3.5kb on FAT32 and 31.5kb on FAT16, because a file always takes at least one allocation unit of space.
Therefore the maps of some countries use up to 3 times as much space on the disk than the actual file size indicated by the Nokia maploader. That’s why you’re offered less free space for downloading than you actually have on your SD-card. Nokia assumes that the maps use about 1.4x as much space on your disk compared to their filesize. That’s certainly just an average, but this multiple depends not only on the maps you’re downloading, but also on the way you format your storage card.
Saving space by reformating your micro-SD card
The Nokia N95 supports FAT32 and different allocation unit sizes; so formatting your micro-SD card with FAT32 and 512 bytes unit size saves you a lot of storage; the additional space used for the FAT (File Allocation Table, where the contents of a storage device are listed) , about 1 Meg, doesn’t really matter compared to the space you can save with the maps. With this format, you can save about 50% of space required for maps. But – transfer times from and to the storage card are about doubled. So depending on the maps you want to download, you might also have good results with 2 or 4 kb of allocation unit size. The rule is: “empty” parts of the map covering rural areas produce small map files, small map files waste storage, when using only parts of a full allocation unit; that’s why Algeria uses about 3x as much space on the disk compared to the actual filesize, while dense areas such as Singapore, Hongkong and Northrhein-Westphalia (Germany) barely need any additional space on the storage card.
Formating an SD-card is possible only with a card reader (I think; if I’m wrong, let me know); no need to say, that you make a backup of your card before formating it..
Optimizing transfer-times from hard-disk to the SD-card
One more hint: USE A CARD READER! Transferring maps to the card with a cardreader is about 3-10x faster to a card reader than to your mobile connected via USB. Card readers are really cheap these days and are definitely worth it.