Well, the good news is that a lot of the infrastructure
required for managing off-line or remote images is already in place,
but it's either hidden or not very elegantly presented. Nevertheless,
by using this infrastructure it's fairly easy to reference off-line
images, and get them back into the library again when required. This
same infrastructure can also be used as a pseudo-synchronised library
(i.e. a Unitary Library).
Important Background Information
on Lightroom Library
A Library folder with the default name Lightroom is
automatically created the first time that the application is launched,
and will most likely be located on your boot disk at
Users/username/Pictures (Mac OSX) or My Pictures\Lightroom (Windows
XP). A quick check of this folder will probably show a number of files,
subfolders or packages, but for the purposes of this tutorial we're
only really interested in the managed photos folder and the file named
- Lightroom Library.aglib.
The Managed Photos folder, will contain any images that
may already have been imported into Lightroom via the Copy, Move or
Copy as DNG options. Since this folder usually contains images
associated with the managed library this folder is usually
referred to as the Managed Photos folder. Actually, if you do have the
Managed Photos subfolder inside the Lightroom folder and it contains
images, then now is a good time to drag it out. Obviously this may
break the links between the images and thumbnails, but I will show how
this can be fixed later. Once moved it's also important that the
application knows that the Managed Photos folder is now located outside
of the Lightroom folder. Therefore, I suggest that you update the
Lightroom Preferences as demonstrated by the example shown in Figure 1
below.

Figure 1 - Telling Lightroom where to place Managed
Photos
The Managed Photos folder can be located pretty much
anywhere, which includes external disk drives or even a network server.
The folder can even given a name more in keeping with your normal
folder naming convention. The important point to keep in mind here is,
that by keeping the Managed Photos folder outside of the Lightroom
folder you are making it much easier to transport and subsequently
back-up the library independently of your images.

Figure 2 - Lightroom with existing image Library
Step 1 - Moving the Library
At present, the simplest approach to running Lightroom
on multiple computers is to have only one Lightroom library (i.e.
Unitary Library), but share it amongst these computers. For this
purpose I use a
SmartDisk FireLite 100GB FireWire drive, but any compact
portable disk drive is suitable. This particular disk drive is powered
by the FireWire port of my Apple PowerBook, is compact and lightweight.
USB 2 versions are also available. At a spin speed of 5200 rpm it's not
the fastest disk drive in the world, but not having to connect it to an
electrical supply is a big advantage when working in the field.

SmartDisk FireLite Portable Disk Drive
The Unitary Library (i.e. the folder named Lightroom
and its contents) should be placed onto a portable disk drive (see
figure 3 below). To do so simply move the Lightroom folder from the
computer disk drive across to the portable disk drive. If the library
is new it will take only a few seconds to move. However, if you have
already imported some images into Lightroom it will take a little
longer. The actual time taken depends upon the number of thumbnails
stored in the Library. So, relocating the library onto the portable
disk while it's still relatively small has its advantages.
It's also worth mentioning that, if space is tight on
your laptop it might be worthwhile using the portable disk drive as
your image store for work-in-progress images. Likewise, any images that
you need to access regularly when away from your main workstation. I'm
not suggesting it as a permanent store, but one that is used for short
term portable storage. Once you have access to the desktop computer the
image folder can be moved across to more secure and faster disk drives.

Figure 3 - Moving the Lightroom Library to a
Portable Disk Drive
Step 2 - Accessing the relocated
Library
Lightroom Beta 4 can still only access one library at a
time, so accessing the version now installed on the portable disk drive
requires a little trickery.
-
Hold down the Option (Mac OS) or Ctrl (Win
XP) key and launch Lightroom.
-
Browse to the portable disk drive
-
Open the Lightroom folder and select the file named
Lightroom Library.aglib (Note: the file extension isn't
normally visible in Win XP)
-
Hit the Choose button and presto you have
reactivated your old Library (see Figure 4).

Figure 4 - Selecting the relocated Library
Note: this description for relocating the
library was first published by Michael Reichmann at
Luminous Landscape
When Lightroom reopens you may
find that your images now have a "?" symbol on the top right
corner (Figure 5 below). If the "?" mark symbol is present
then this is a sign that the links between the library thumbnails and
actual images have been broken and that you'll need to repair them.
However, more often than not Lightroom will reopen with the links back
to the original images still in place. If the links are still intact
the thumbnails will look as they do in Figure 2 above (i.e. no "?"
symbol).

Figure 5 - Lightroom has lost the link to your
images
Images with the "?"
symbol in the top right corner are orphaned and cannot be edited in
Lightroom until re-linked with the library. However, so long as the
original thumbnail process was allowed to complete, then it's
still possible to apply Keywords, Ratings. etc. Likewise, it's also
possible to create and present slideshows and web galleries. It's even
possible to print contact sheets.
Earlier I indicated
that Lightroom could only access one library, but this should not be
construed as meaning that you can have only one library. Actually, you
can have as many libraries as you wish. Space permitting, they can all
be stored on the same portable disk, although each must have a unique
name. To open a alternative library simply hold down the Option
key (Mac) or Ctrl key (Windows)
when launching Lightroom and a dialog similar to the following will be
displayed.

Figure 6 - Picking a Library
Step 3 - Locating your images
In the event that the links between the library and
images were broken when you moved the Lightroom folder, then
re-establishing these links is the next step. Finding the images again
is actually very simple.

Figure 7 - Finding your Images
-
If the identified disk drive is off-line you should
reconnect it. Generally, this is all that is needed for Lightroom to
automatically re-establish the link and make the "?"
disappear. If it doesn't then hit the Locate button -
Lightroom will immediately open an OS dialog and highlight the
relevant image (see Figure 8).

Figure 8 - Selecting the Image
-
If Lightroom doesn't automatically locate the image
then you will have to do it manually using Folder or List view as
shown above. It's important that you find and select the same image
as was selected in the Lightroom window otherwise the previews could
easily become corrupted.
-
Once the correct image is found hit the Select
button and the dialog will close.
On return to the Lightroom you'll notice that the "?"
symbols begins to disappear from the thumbs. Once the "?"
disappears it will be possible to begin editing the images within the
Develop module.
If you disconnect the portable disk Lightroom will
loose track of where the library was and will display a dialog similar
to the following when launching the application:

Figure 7 - Lightroom has lost the Library
As can be seen in the screenshot Lightroom advises you
of where the Library was last located, which is generally all that is
needed when using the Mac platform. However, Windows seems a little
more prone to loosing the Library when the user moves it using
Explorer. If this occurs, then you will need to use the Locate
Library button to navigate to the new library location. The
Use Default Library option creates a completely new library, and
should only be used when that is what you want to do.
If you've got this far then you'll be glad to know that
all the difficult steps are finished. I've demonstrated how it's
possible to relocate a Lightroom Library and subsequently re-link any
orphaned images. However, it's also possible to do it the other way
round. That is to say, relocate the image folders. The process of
finding archived or even orphaned images is the same as described
above. For example, say the original folder with images has also been
relocated to an off-line storage device or DVD. Lightroom will identify
all such images in the same way as I described above (i.e. the "?"
symbol in the top right corner of the thumbnail), which when clicked
opens the same dialog shown in figure 7 above. Again, the dialog
includes the name of the volume, folder and image name, so finding the
correct disk should be relatively easy. Once the disk drive is
reconnected Lightroom will automatically re-establish the links between
the thumbnails and all of images stored on the disk.
Finally, by combining the principal of Unitary Library
and Off-line storage as described above you can manage very large
numbers of images in multiple locations. Hopefully, the methods
described above will be made redundant in the near future, but in the
meantime it's probably the easiest way to manage and access large
numbers of images quickly.