I switched over to a Mac Mini last year after years of Windows computers. Since I had nearly 2 TB of photos spread across 3 hard drives, I bought a new WD 3TB hard drive and formatted for the Mac and then moved my photos to the new external HD from my old one. It went easy. One advantage I had was my old external drive was formatted in FAT 32. I then had to tell Lightroom on my Mac where my photos were. The best help I got was from Lightroom Queen Victoria Brampton's site. She has tutorials on how to move your Lightroom catalog to new computers. It wasn't quick, mostly because my old catalog files were all over the place on my Windows computer. I first had to straighten out my mess on my Windows computer.
The Lightroom Queen - Tips, Tutorials & Troubleshooting for Adobe Photoshop Lightroom