Picasa at times may not scan or recognize certain drive letters in the Folder Manager under the Tools menu. This makes it impossible to set the drive to "Scan Always." The problem seems to occur for drives formatted as exFAT and ReFS file systems. Another cause may be that your drive has corruption.
To see if your drive is being scanned (watched) by Picasa, you can open the "watchedfolders.txt" file and look for the drive.
Copy/paste this path "%LocalAppData%\Google\Picasa2Albums" (without quotes) into the address bar of Windows File Explorer then hit the Enter key
The Picasa2Albums folder will open
Inside that folder you will find the watchedfolders.txt file
Open the text file to see the drives and folders that are being scanned (watched) by Picasa
Close the text file
If you did not see your drive listed, follow the directions below.
How to add your drive to the "watchedfolders.txt" file:
Press the Windows key on your keyboard (or click the Start button)
Type "computer management" (without quotes) then click the Computer Management desktop app
Computer Management opens
In the left panel, under "Storage" click "Disk Management"
Wait a short time for Disk Management to load
Right click on the exFAT drive, select "Change Drive Letter and Paths"
In the pop-up window, click "add." You can keep the old drive letter that another program may be using.
In another pop-up window, click "Browse" then create a "new folder" in a NTFS/FAT partition with the name of your liking. It is just a link, so it won't take up any space.
In Picasa's Folder Manager, browse to the NTFS/FAT drive and select the folder you just created.
Set Picasa to "Scan Always" then click OK.
Advanced users could try an alternative solution by creating a mount point for the drive in another file system: