It is your choice and you can change your mind whenever you'd like as to which folder and or sub-folder it will scan...
Maybe when you began using Picasa you thought you had to have Picasa Scan/See all your photos - Now you only want Picasa to Scan/see specific folders or sub-folders you want to work on - Then simply change those folders in Picasa's Folder Manager..
Each time Picasa is opened, Picasa will scan the computer to find the folders or sub-folders you have chosen (and photos with the compatible photo file types you choose) and display only the photos in those specific folders or sub-folders.
Photos never move to Picasa, the original photos stay in their original folder locations.
To modify which folders or sub-folders Picasa is Scanning/Seeing from your hard drive or Portable Hard Drives/memory Card or Flash Drives - use these steps once you are in Picasa:
1. Click the menu in Picasa.
2. Select .
3. Select a folder from the folder list on the left that you want to add or remove from Picasa's watch list.
4. Select one of the following monitoring options on the right:
· “Scan Always”: Picasa views all compatible image types in these folders, and it automatically adds new photos. Picasa actively scans this folder for changes.
· “Scan Once”: Picasa accesses all compatible image types in these folders only once. It doesn't detect new photos added to folders with this setting.
· “Remove from Picasa': Picasa doesn't look in folders that are marked with this status.
5. Click the button to save your changes.
The folders that you've set Picasa to continuously scan are listed in the 'Watched Folders' section.
You can also set a folder to 'Remove from Picasa' by right-clicking (Windows) or Control-clicking (Mac) the folder in Picasa and selecting “Remove from Picasa.”
Remember-you can always change your mind tomorrow on what you've set this setting to by simply repeating the steps and changing your choices...