According to all the posts with PC connection problems this is a weak point of the Galaxy S.
A new issue (at least I did not see it mentioned) is the problem of giving an empty (external) SD (card) folder in Mass Storage mode,
Connecting the phone, set as Mass Storage device, and after mounting (via the notification area) I see the SDcard as a drive. But, as said, the SD folder (the external SD card) is empty.
Is this a common issue and is there a solution I overlooked (I have Windows XP SP3).
A solution what worked is to set the phone as a MTP device (select "connect to Samsung Kies" on the phone).
Then the phone is visible as GS-I9000 on the computer and both cards are accesable.
By the way I do't start Samsung Kies because Kies is not recognizing this phone (as often mentioned by others).