Here is an answer from another forum (original poster unknown to me):
The "preparing" stage in Time Machine
is usually fast, because Time Machine
uses a log of file system activities
to know where to find changes that
need to be backed up.
But there are things you can do to
make that log untrustworthy. When Time
Machine discovers the log MIGHT not be
valid, it does a full scan of your
file system to determine what really
needs to be backed up. This can take a
while. On my Powerbook (and with a
firewire external Time Machine drive)
it can result in a "preparing" stage
that lasts 20 minutes. If you let Time
Machine continue, it will eventually
figure out what really needs to be
added to the backup disk and will
continue. This, of course, is better
than just gambling the log is correct
or just backing up EVERYTHING again
even though only a few files might
actually have changed (which would
take much longer and consume more Time
Machine disk space).
In these early days of Leopard
installs, one of the most common
things you can do that will make the
log untrustworthy is booting from the
Leopard install DVD. Each time you do
that, the next time Time Machine runs
it will discover that your hard drive
MIGHT have been modified without the
changes being reflected in the log. So
it does the full scan -- a long
"preparing" stage -- to insure it gets
things right.
Best Answer
iPhoto puts all of your photos into a library similar to the way iTunes puts all of your music into a music in a library folder structure. That being said you could easily just copy the library from your hard drive to another. I know you want to back it up, but here is some information on how to move the library and much of the info should be useful in your soluition: http://basics4mac.com/article.php/move_iphoto_lib and http://www.hinkty.com/blogger/2006/02/how-to-move-your-iphoto-library-to.html
Also if you want to share a library between users/systems see here: http://forums.macrumors.com/showthread.php?t=158248
And lastly there is a application manager which can handle much of this in a more simplier manner called iPhoto Library Manager