When you spend a lot of time taking and processing your photos, you don't really want to risk the chance of all your hard work being lost, so making backups of your photos is very important.
There are various different ways you can backup your photos (and other data), in this post I'll try and (very) briefly go over the different methods and their advantages and disadvantages. Of course, I'll mention my preferred method of backup as well.
This article covers the different backup hardware available rather than the different types of backup and backup comparison that you can do. For information on that, the Wikipedia article on backup offers a reasonably good yet concise explanation. For my backups I use a differential mirror backup to only backup files that have changed, while deleting any files on the backup that have been moved/deleted on my main hard drive. Please note that the article is written with this backup method in mind.
Network Attached Storage is basically a small specialised computer, typically with 2-4 drive bays. Some units come with hard drives pre-installed, others you buy the drives and install them into the NAS unit yourself. The NAS unit is usually connected to your router (though you probably could connect one directly to your PC) using standard network cable.
Network Attached Storage has a couple of disadvantages though. First the NAS unit is typically quite expensive (e.g. ~£150 for the NETGEAR RN10400-100EUS).
The second disadvantage of Network Attached Storage is that the transfer speed using NAS is likely to be quite a bit slower compared to using internal hard drives.
RAID (or RAID1 to be more specific) is where you mirror data across multiple disks on your computer. So you have to double up your disk space, e.g. if you had one 1.5TB drive, you would need another 1.5TB drive to mirror the data across. For more security you can actually mirror the data over more than two drives.
The main benefit of RAID is that if one of your drives fails, you can carry on working as normal. Just replace the failed drive, and RAID will take care of mirroring the data, no need for backups or restores. And of course, all data is up to the minute, not from when you did your last backup.
The disadvantage of RAID is that if your computer receives a very high voltage e.g. lightning strike, or sets on fire, you may loose all your drives, and so all your work. Also, if you somehow manage to irrecoverably delete all your files, this would be automatically mirrored to your other drive, and so you would have no backup.
For the full low-down on RAID, you can, of course, check the article on RAID at Wikipedia.
External hard drives usually cost around the same as an internal drive (sometimes a bit more, occasionally a bit less).
Regarding transfer speeds, the main bottleneck with an external drive is how it's connected to the computer. USB or Firewire drives are relatively slow (though they may be fine, depending on how much data you have to backup and how long you mind waiting for the backup to complete). If you have an external drive that you can connect using e-SATA, it will likely be as fast an internal drive.
DVD writers are cheap, and Blu-ray writers aren't massively expensive either. Writeable DVDs are very cheap, but writeable Blu-rays not so much. The main problems with using DVDs and Blu-rays for backup is that your backups will have to be split over multiple disks. This means backups take longer as you have to switch out the disc when it is full, and of course the same thing should you need to do a restore.
Another problem is that whenever you change a file it will need to be backed up, and so you may end up with multiple versions of a file you have changed a few times spread over various discs. Of course, this could be seen as a good thing since it means if you mess up an photo you can easily go back to an older version. It does mean that you can get through discs quite quickly though, and of course meaning you will have more discs to go through if you have to do a restore.
I have seen web backup mentioned as a backup solution in a few places. This is generally a service you subscribe to, where you backup your files to a server over the internet.
Personally, I can't see how this would ever work unless you have an extremely fast upload speed on your internet connection. I have an "up to" 8Mb/s (1MB/s) internet connection, but the upload speed of my connection is only about 32KB/s. So to upload 4GB of data would take 36hr 24min 52s. And that's only if I could keep my upload speed at its maximum and didn't use my internet connection for anything else while the backup was running.
And of course, if you need to do a restore, this is also going to take a lot longer than if you had the data available locally.
You install the drive bay in your computer, then can insert an internal hard drive into the bay, just like you would a CD with a CD drive. In a way it's pretty similar to using an external hard drive, except you plug the whole drive into your computer instead of just a cable.
I have two backup drives for each of my main hard drives, so I can suffer a main drive failure and a backup drive failure. This may seem extreme, but given the time I've put into my photos, I think this is worth it.
I bought my SATA hot swap drive bay from Deal Extreme.
Not a backup method, but a UPS can help prevent data loss. I have a UPS between my computer and the power, this means that if there is a power supply or spike, the UPS should keep the computer going okay. While I haven't suffered any real power cuts since purchasing the UPS, there have been a few sub-second power cuts that have caused the other PCs in the house to reset while all I got was a beep from the UPS, saving me from loosing what I was working on.
For backup software I use the free Synkron and also Beyond Compare (which costs $30). I find Synkron is better for mirroring multiple folders in different locations. Using Synkron you can initially set up tabs for each folder you want to backup onto your backup drive. Then you can just use a 'Sync all' command to backup all the folders. Synkron will remember your tabs, so backing up all the folders in future using the 'sync all' command is easy.
One of the problems I have with Synkron is that while it doesn't crash, it does become unresponsive, and often looks as if it crashed until it finishes the backup. I haven't had this problem with Beyond Compare. Beyond Compare also has a binary compare feature, which is useful for finding files/photos that have become corrupted on your hard drive, but are still okay on your backup, enabling you to restore from the backup and replace the corrupted files.
The benefit of this is that otherwise you might not know that you have corrupted files. If you then moved or changed a folder containing these corrupted files, when you did a backup (assuming you are doing a mirrored backup), the folder on the backup that contained the uncorrupted files would be removed, and the moved folder containing the corrupted files from the main hard drive would be copied to the backup.
This has happened to me when I moved all my photos to a larger drive. When moving the files over, unbeknownst to me, some of the files weren't copied properly and became corrupted (at least I assume this is when the files got corrupted). I had bought two new hard drives for backup that matched the same size as the new drive, then performed a backup from the new drive to the new backup drives (which of course copied over the corrupted files as well). By the time I found that some of the images had become corrupted, I had formatted my old drives and was using them for something else, so there was no way to restore the uncorrupted files.
As a result, I'm stuck with quite a few corrupted images in my photo library that I can never get back. If I had been able to do a binary comparison after first copying the files to the new disk, I would have seen that some of the files didn't match (due to being corrupted), and be able to overwrite them with the uncorrupted versions. Binary comparison is very slow though, so I would only recommend to do a binary comparison every once in a while.