After you've dragged files in a Sagan, you'll notice a number appear in the top right corner. The File Upload Indicator shows you the number of files still yet to be uploaded to the cloud (your Sagan Drive). You'll notice the number go up as files are added to a Sagan from an external file explorer/Finder, and down as files are uploaded to the Sagan Drive.
Leaving the page while this number is still present may result in lost data. If possible, don't close the browser until the upload has finished (and the indicator has gone away). If there are uploads still happening, you should also receive a warning when trying to exit:
For more info on adding your files to Sagans, check out this video: