I've got the same problem and I've done a fair bit of testing, without any real solution.
I've repaired the database, and that made no difference. I've started with a blank photo.library and that made no difference.
One of the tests is this:
Insert an SD card or camera (via USB), then Aperture 3.6 (A3.6) starts up itself, and the import routine starts. It says "Loading Assets". If you click on the swirling circular icon, a window pops up with the Activity. You can compare the progress of file importing in this Activity window with the number of Imported files notified at the bottom right of A3.6, which also says number of files imported.
It should go through the SD card and complete the task and the activity ends.
However, instead, randomly, it often gets stuck, part way.
When I eject the SD card in Finder and reassert it, the routine starts up again.
I've done this repeatedly, with my Photo.Library and a blank database/library. I've used two SD cards, on 1Gb the other 2Gb.
I've tried it with A3.6 running and I've tried it by closing A3.6 each time and starting the program fresh.
Each time I get random results. It starts the import and freezes after a random number of photos. Sometimes, it stops after just 6 photos, sometime its stops after 156. Occasionally, it completes the task and does all photos (on these SD cards that's about 220 or 415 photos).
There seems to be no rhyme or reason.
In 10 recorded tests (out of many unrecorded), it completed as follows:
1) 155 photos imported
2) 176
3) 136
4) 156
5) All 415 photos, task completed
6) 256
7) All photos, task completed
8) All photos, task completed
9) 6 photos
10) All photos, task completed
APPLE - PLEASE RESPOND. YOUR REPUTATION IS AT RISK AND USERS ARE LOOKING TO DUMP APERTURE. Some on this forum think you are ignoring this problem, or even creating the problem, deliberately.
PLEASE RESPOND....