How should I backup my hard drive with several partitions to an external drive?

First let me say good on you for setting up a backup. Sadly you’re
in a minority. Most people don’t think about backups until it’s too late.

The answer to your question depends in part on the capabilities of your
backup software. But I do have some ideas and recommendations.

My experience is that most backup software will read each partition as a
separate entity. You probably have different drive letters assigned to each of
your partitions, and most backup software likes to deal with those “logical
drives” separately even though they might reside on the same “physical
drive”.

But it’s also very likely that your backup drive does not need to match the
partitioning on the primary drive. There are times when you might want it to,
which I’ll explain in a minute, but more commonly the backup programs will
backup everything in to single massive backup file or folder, perhaps
with daily incremental additions.

Let’s take my system as an example.

My entire “C:” drive is backed up once a month using Acronis TrueImage Home to a single file on my “E:” drive.
“Leo-Monthly.tib” is, today, a 53 gigabyte file. Every night a new file gets
generated, “Leo-Monthly##.tib” where the “##” is replaced by an increasing
number. That nightly file simply contains the updates to the backup from the
previous day. They vary in size from 2 to 10 gigabytes, depending on how much
changed. At the end of the month, the process starts over.

The point here is that there’s nothing special about the
way I’ve partitioned my backup E drive. It’s just a drive that’s big enough to holding
the backup files. Some really big files.

Now, other backup approaches may work differently.

Let’s say that instead of using a true backup program, you run a batch file
every night that simply copies the contents of your four partitions to
your backup hard drive. You’ll want a way to keep track of each partition on
the backup drive. One way would be to create matching partitions, I suppose.
But another, more flexible approach might be to simply create a single
partition, and then create a sub-folder for each of the logical drives that
you’re backing up.

It’s quite possible that some backup programs might well work in a similar
way.

The one case where you might want to set up identical partitions is if you
expect to be able to swap the backup drive in as the primary if the primary
fails.

Most backup software assume that you’ll repair or replace a broken drive and
restore your data to it from your backup copies. That means that the layout of
the backup drive is fairly unimportant. However if your recovery
approach is to physically swap drives, then yes, I would expect a number of
additional constraints would have to be met, partition layout being only one of
them. This is an approach I recommend only for advanced folks.

Imaging software that you mention is another approach. In fact, true imaging
software is probably the best way to achieve that scenario where you can
recover by simply swapping drives. Imaging software of this ilk should simply
“do it all” – meaning that it’ll take care of replicating the partitions and
whatever else from one drive to another.

Not all imaging software works that way. In fact, it often works in a manner
very similar to traditional backup: the image is saved to a file. That file can
simply be saved on any drive that has enough room. When the time comes to
restore, the image is simply placed back on to the replacement drive.

Ultimately the decision of how to best partition your backup drive depends
on exactly what tools and techniques you use to backup. Most likely you’ll need
to do nothing special. I typically recommend partitioning it as a single drive.
But you’ll need to check the documentation associated with your backup program
to be sure.