I'm running out of space for image files so instead of adding another drive and splitting them I've ordered a much larger drive and will migrate all the files onto the new one. (don't worry I have ordered 2 1TB drives, 1 to be used for backups)
I have a few lightroom catalogs (I prefer this to 1 large catalog) and want to retain all my flags, keywording, manipulations etc.
I would moves all of your image files and folders over to the new hard drive with exactly the same heirachy that they currently have.
Lightroom will fail to find any of the images when you first open it after the migration, but you have the option to 'locate missing files or folders' - find them on the new hard drive and you should be good to go.
If you have all your images inside one folder with many subs folders ie. "My Pictures" locating this finder should then relink all of your sub folders too. If not you will have to relocate each one individually.