Thanks to help from this site managed to convince my HG10 to download to iMovie 8 on a MacBook Pro running 10.5.1. Got back home to lousy UK weather and thought I'd load content to quadcore 3.0ghz MacPro (also 10.5.1). Nope, not a sausage.Camera not recognised, in any USB2 port, nor by FCE 4.0. I tried reloading, rebooting from different hard drives etc. Disc shows on desktop, is scanned by Norton and so on, but programs don't want to know. I've tried different cables as well. Still fine on MBP. XH A1 works fine with FCE and iMovie, but the HG10 is getting more use as it's smaller.
Any ideas? Mystery to me and I've been in the Mac universe now since '89. Part of idea of MacPro was as movie editing centre and it's somewhat disappointing that so far it's NBG.
For FCE mesaage is can't find HD deck, and for iMovie it shows greyed out menus, save only import from camera, and no usual edit screen. These are BNIB products bought from Applestores. Computer memory checks OK, as do all other USB devices and ports when used with other devices. Oddly enough, stills from HG10 load instantly with Canon software.

Why do you need a particular program to recognise the HG10? Can't you copy the files to your HD and then read them from there?

I don't use FCE, but in Final Cut Pro 6, you use the "Log and Transfer" menu item (as opposed to "Log and Capture" like with ordinary video ingests).

Does FCE have that command?

FCE does have "log and transfer". Hitherto it has not recognised that the HG10 was connected - now it does, and I have no idea what caused the change. Still, it works and that's the key.
iMovie 8 apparently has a bug which affects some who upgrade from 7, like me. Fix is to select system preference>colours and then select a different choice from existing one, and then switch back. The fix appeared on iMovie forum on the Apple website, looks inherently unlikely and actually worked for me.
Although I got the HG10 drive on the desktop I couldn't open anything useful in it with FCE or iMove - or for that matter with any other application. Mercifully issue is now academic.