When you imported your images from the external drive to the MBA, where did you put them?
Are they in the Pictures folder?
Unfortunately I suspect when you imported them you chose "Add" as the option at the top of the import dialog. In that case Lightroom will have cataloged the files on the external drive and not moved or copied them to the MBA.
Reformatting the drive will delete the data. That is why Lightroom cannot locate them.
Perform a Finder search for your images on your MBA to be sure they are not on the internal drive.
I'm not sure where LR4 put them. I ususally just import from my external hard drive and then export them back to the external hard drive when I finish editing them. Before I reformatted the hard drive I saved all of my files to the desktop on my PC. Then I reformatted it on the MBA and transfered all of my files from my PC desktop back onto my external hard drive. So, all of the original files should be on there. However, when I plug the hard drive back in and try to import those same files again, it dims the pictures (letting me know that thoses files already exist and that I can't import them).
Ok, so at least you haven't lost the images.
The catalog knows "about" the images but not where they currently are. You cannot import them again because they have already been cataloged.
If the images are on the external drive in the location you want, the the procedure to re-connect is as follows.
Select one of the thumbnails. It should have an "!" or "?" on it (upper right). Click whichever it is and you will be asked to Locate the file. Navigate to the file to re locate it and make sure the check box "Find nearby images" is checked.
Lightroom will now relink all the images.
I usually just import from my external hard drive and then export them back to the external hard drive when I finish editing them.
this is not a "correct" workflow when working with catalogs. The files remain on the external drive. The act of importing is only necessary once to catalog where the files are and to allow quick access to them via the Library. Editing them either in or outside of Lightroom via Photoshop does not require you to move the file.
Thank you so much David. I was able to relocate all of the files when my hard drive was connected. I wanted to ask you one more thing. I reformatted my Seagate hard drive so that I can use it both with the mac and pc. However, when I connect it with a pc it no longer recognizes it. Any suggestions?
Good. Images back and connected.
As for the drive. If you Used Disk Utility on the Mac and Formatted with MS-DOS(FAT) I have no explanation as to why you cannot see it on a PC.