Master Image Offline - HELP

I would try this first:
Click on "FILE" in the menu bar and pull down to "MANAGE REFERENCED FILES". In the top box you will see your photo highlighted. In the bottom box, navigate to the same photo. Click on it and then press the "Reconnect" button at the bottom of the dialog box.

If that does not work, I would delete it from Aperture and import it once again. Be sure to just delete the information that Aperture stores about the photo and don't allow Aperture to move the original to the trash.

I have the same problem and your solution won't help : the "managed referenced files" dialog shows the main HD as "offline". Selecting it then clicking on "mount" doesn't change anything, and the files, even though marked as not accessible, *are not listed as such* in the dialog (thus there's no way to reconnect them). It seems Aperture somehow puts itself in this inconsistent state, and I can't find a sure way to fix it (though restarting sometimes helps but not always).

Quite a serious bug IMHO.

Move the files to another volume. Then try reconnecting.

Check that the hard drive is formatted in "mac os extended"... APerture has a hard time referencing masters to MS DOS Formatted drives, and unfortunately most external drives use that format when purchased.
It is a pity that the OS doesn't display an informative (or even a warning) when those disks show up on the desktop, because really for many apps support of MS DOS formatted drives give us some quirks, & or minor problems...

I have the exact same problem. I've spent well over 1 1/2 hours with lame techies who were of no help at all. This morning I had to restore my computer to a few days ago, before I was having operating system problems. I imported my aperture vault from my external hard drive and then once this was opened I see that all of my photos show as offline and not linked, despite the fact that they reside on the Pictures folder on the Mac HD. I excluded the Aperture Library from the Time Machine backup.

Anyway I can only successfully reconnect in the Manage Referenced files dialog box if I do each file individually. Not exactly an option considering the volume. I am hating Leopard and Aperture at the moment.