Sony is stealing memory from your MS-duo by the way they format the memory stick on the P800. When they format the stick they do this by using a cluster size of 16384 bytes which really gives a lot of slack space.
This means that a file which is only 100 bytes in size will take up 16,4 kilobytes on your MS-duo, which really is a waste of memory.
So when you have the possibility to format the stick on a memory card reader on your PC then do so. It will save you a lot of memory.
Below you see a table of formatting the stick with different cluster sizes and the free memory you will have when you install about 7220k of data.
Cluster size/ Free / Total
1024 / 8111k / 15717k
2048 / 7746k / 15748k / (my preference for 16M and 32M MS-duo)
4096 / 6904k / 15764k / (my preference for 64M and 128M MS-duo)
8192 / 5128k / 15768k
16384 / 1392k / 15776k / (This is the Sony default)
There is however one disadvantage to decreasing the cluster size and that is speed degradation. Therefor my advice is to format your MS-duo with a cluster size of 2048 or 4096 which gives a good balance between free memory and speed.
Notes:
1 This trick will also work with on a 32M, 64M, and 128M MS-duo
2 The increase in free memory will be a lot less when you have only large files on your MS-duo
First make a backup before you start with this
To format the MS-duo on your PC use the following format command in a command window.
FORMAT S: /A:2048 /X
or
FORMAT S: /A:4096 /X
where S: is the drive letter