I just got a macbook and bought parallels and a copy of windowsXP primarily to be able to run my Garmin GPS mapping software. Everything works fine until I try to plug in my GPS. Parallels acknowledges the Garmin GPS. But soon after that acknowledgment I get this: No other app is open when I plug in the GPS. I've tried with the mapsource program both open and closed. I can't get the GPS to come up on the virtual machine!
Which Garmin device? For mine, I had to download a new version of driver for windows XP which resolved the issue.
Don't forget to download the Mac version of Garmin web updater. This will insure your 276C is running the latest firmware. I know I had to do a firmware update on mine (Quest) before I could get it to connect reliably to mac and windows. But in my case, I am lucky enough I have some old XP laptop hanging around for such necessary pains.