Canon does recommend using your external power supply when connecting to the computer (which I never do, of course). As long as your batteries are good, that shouldn't be a problem (note: if you haven't done so, go get yourself a good set of high-Ah rechargeable batteries, something in the 1800 Ah range).
Just be happy that OS X recognizes your camera... mine did for awhile, but hasn't for months (in fact, it locks up my system, requires a reboot!).
There's also the possibility that your 128mb CF card is a bit wonky. Once you succeed in getting the photos off the camera (perhaps with a USB CF reader, which are pretty cheap), reformat the card using the A40's menu (not from the computer, that is). That might clear things up.
Also - A40s of a certain serial number range required a firmware update and a "focus fix" updated, both available from Canon's support site.
Finally - join the Canon PowerShot forum:
...where all of your Canon camera questions will be answered...