I located at least part of the source of the problems with installation of the maps.
The Map Loader creates thousands of small files and the disk space they occupy is far more than predicted due to the file slacks (the difference between the size of the data in the file and the minimum file size of 1 cluster).
I initially overlooked these files because I knew my memory card was full and so was searching for (and not finding) a folder containing a large amount of data - the reason is that many utilities report the disk space used by the file contents without also reporting the space occupied on disk (or when it did list both my eyes locked only on the first figure).
This problem was compounded by the fact that at some time some of these files on my memory card became corrupted (perhaps during one of the deliberate or accidental disconnections while the Map Downloader was still trying to work). I had managed to partially install the China maps so I could see major roads in China in the phone but Map Downloader did not indicate that China maps were installed.
As a result of this file corruption I was not able to delete the maps with the Map Downloader and nor was I able to uninstall smart2go.
The remedy was to find and delete the diskcache folder concerned which on my card was
/private/20001f*/diskcache
(I used a wildcard to partially disguise the folder name above in case it is user specific)
Some more detail
I installed Smart2Go on my 256MB media card.
When I first launched Smart2Go (without a network connection) the /private/20001f* folder is populated with 272 folders, but only 5 files amounting to a total 484 bytes (size on disk 48 KB).
Map Loader inaccurately showed that space on my device was 120.5 MB
[Memory card app says 168 MB free]
China maps are listed as 18.5MB, therefore it seems reasonable to think that there is space to install them in the 168MB of free space [Wrong!]
As soon as I start to download China maps the "Space left on device" listed by Map Downloader changes from the original value of 120.5MB to 101.9MB - which is consistent with the 18.5MB alleged size of the China maps. It does not decrease as installation proceeds.
The download then starts with an initial estimate usually 2 hours, sometimes 1 hour.
It actually takes longer than this and then over-runs at the end, perhaps because the media card is full. Inevitably I am forced to break the connection and kill the Map Loader process.
At this point, examining the /private/20001f*/diskcache folder shows it contains 10,625 files totalling 15.3MB but which occupy 169MB of space due to the file slacks.
This equates to about 15,240 bytes wasted space per file. I guess the clusters are 16k in size and the actual data in each is only about 144 bytes (after all there is very little detail in the China maps). Therefore, extrapolating the 15.3MB installed to the 18.5MB claimed size I predict I would need about 204MB of free space on my media card to install the China maps.
Speculations
Probably the amount of wasted space is less if the map is more detailed but the total space consumed will be no less (I suspect the number of files is related to the geographical area of the map - some kind of tiling scheme).
Probably it would be possible to reduce the space wasted by reducing the cluster size with which the media card is formatted (my card was as provided by Nokia - do they have a good reason for using large cluster sizes? - I understood disk fragmentation was not an issue for memory cards because they don't suffer the slow access times of hard disks).
Perhaps also internal phone memory is more efficient in storing data.
It occurred to me the very slow installation speed might be related to all the file operations that are involved in creating so many files. I temporarily changed the profile for USB disks to "Optimize for performance" instead of "Optimize for quick removal" in the Hardware device manager, but I did not make any detailed timing comparison and it still takes hours even with that setting (and of course it increases the risk of corruption when you deliberately or accidentally break the connnection).
I don't know why deleting maps was so slow. Currently deleting "All maps" to remove the partially installed China maps seems to take a few tens of seconds but earlier it had seemed to take tens of minutes, and when the files were corrupted, it would apparently hang.
Advice for people considering installing maps is
Consider formatting memory cards with small cluster sizes to minimize space wastage.
Figure on much more memory being used than predicted by the Map Downloader (more than 10x more in my case).
Be prepared for a long time to install (or uninstall) - expect it to run overnight. Don't start if you will need to interrupt it in a few hours.
Make sure that the phone battery is well charged or it is on the charger.
As for me - well it's back to Pathaway running on my Sony Clie TH55 - I don't think these maps are yet good enough to consider getting a bigger memory card or going to the trouble of reformatting this one. It would also help if the software was more mature - as in correctly estimating the amount of disk space it will take to install.
Better still, tiling the way Pathaway does - within a single file so there is no issue of space wasted in file slacks.