Some time ago, when writing about Aperture 2, I mentioned that:
The other slight wrinkle was that it was difficult to tell when Aperture had finished erasing a card. In Aperture 1.5, the project would continue to show the import progress spinner until the card was erased and ejected. As soon as you saw the project shoebox icon, you knew that the card could be yanked. I need to do a little more testing, because now I’m not sure how you know with Aperture 2 - it shows the shoebox icon as soon as it starts erasing the card, not when it finishes. That was really the reason why I ended up with partially-erased cards.
Jim's suggestion was to use the Growl add-on HardwareGrowler to be notified when Aperture finishes erasing a card and unmounts the volume. A lovely, simple tip.
As a bonus, if you install Growl, FlickrExport will give you upload progress notifications.