The problem is, you moved/saved the photos to the hard disk outside of LR... and now the catalogue's internal links to where those photos are is broken...
You have three choices... depending on the number of photos involved...
you can use LR's internal functions to find/locate the photos, and fix the links..
or
You can create a new catalogue, then point it at the new hard drive, and let LR do a complete NEW Import of the photos in their new locations...
Option #1 may take days of tedious work
Option #2 won't carry over any editing changes you made...which are stored in the catalogue unless you diligently created XMP files...
Third option is to copy all the photos back to their original locations (you should have named/partition the new apple hard drive the same as your old PC hard drive) and be sure the original catalogue is working, then use LR to move everything to the new hard drive... LR will move the photos, and keep track, properly, of the new location in the catalogue...