I have recently moved from pc to mac and am about to start backing up my mac to a separate hard drive using Time Machine. My question is, if I am in the unfortunate position of needing to use the back up to restore Lightroom (both images and catalog), will the Time Machine restore successfully marry the individual images with their processing which is held in the catalog? Or would it be better to partition the separate drive and also export my Lightroom catalog and images to it - this was the method used to move from the pc, and worked very well. Help appreciated.
Lightroom edits exist in the Lightroom catalog*, the image themselves are untouched. Edits can, optionally, also be recorded in XMP sidecar files, which are files associated with the RAW images and typically are stored in the same directory as the images. Where your Lightroom catalog exists is dependent on where you decided it should be, as Lightroom doesn't care.
Lightroom handles image location changes every elegantly, and isn't all that bothered when it can not find the image files, which you will know as the Library module will show a series of "?" showing its not sure where the files are. If you restore images in a different location, or on a different drive, you simply tell Lightroom where those images are (right click on a folder and choose "update folder location") and Lightroom will update its links to those images.
This makes it very simple to move your images to other harddrives, rename a hard drive, whatever, as its only a one or two click change in Lightroom.
So, I wouldn't worry about 'marrying' your catalog and images, as this is a standard, simple task in Lightroom. Worry more about keeping backups (note the plural) of both your LR catalog and the images.
**technically in metadata, which is what is stored in the Lightroom catalog, as well as in other image types like DNG and PSD, which have provisions for storing metadata in the file itself. 'XMP sidecar' is a metadata file*
What do you mean by "start backing up my mac to a separate hard drive"? If you backup everything, then you won't need to do any marrying, because everything exists on the backup. If the backup stores the catalog, and the images, you will have the adjustments made in lightroom stored in the catalog.
You also can use xmp sidecar files - so if you are using those I would suggest editing your question to describe how. Lightroom automatically writes adjustments and settings metadata to the catalog. You can also have Lightroom write changes to XMP. If you want the edits visible in other applications, you need the XMP sidecars anyways. So basically, you can have all of your edits in the catalog, in the sidecar files, or in both. So your backup solution needs to consider this.
I think the main question you need to resolve is, what is time machine backing up?
I use lightroom and back up to time machine, and things work fine. If you need to do a restore and some of the connections break for some reason, they can be hooked back together pretty quickly.
I don't think time machine is good enough for backups of a large photo collection. My experience is it's fine for recovering individual files or a set of folders, but restoring an entire disk from Time Machine can get really painful, especially if you're doing backups over the net instead of to a firewire disk (even USB 2.0 is painfully slow to recover a large disk entirely, IMHO).
So I do more than just Time Machine. Belt and suspenders, maybe, but the more copies of your data, the better. You can read the full details here (http://www.chuqui.com/2011/07/backing-up-your-data-the-2011-edition/) but basically, I use SuperDuper! to make bootable clones of the boot drive in case of catastrophic failure, and Time Machine so I can get to recent versions of individual files. And my secondary data is on a NAS, which is mirrored RAID, so there are multiple copies. And I am pretty religious about keeping off site copies of everything. My end result is that I have about 5 copies of key files, and three copies of files I can "lose without being too freaked"). Which may seem overkill, until you can't get a copy of that file back...
As inexpensive as 1 and 2 terabyte drives are now, and as important data is, I think it's critical to get into the good habits of doing backups, because it's not expensive, doesn't take a lot of time (once you get into those habits), but the problems of not doing it are really painful....