I found this in the knowledge base, no wonder people keep using them and are un aware that they generally stopped being dependable a few years ago. Why does Cakewalk still recommend them as a back up? 

Tips on Backing Up - Bundle Files

Backing up your Cakewalk files - The benefits of the Bundle
In the wonderful world of computers, backing up your work is more than a good idea. It's essential when you're using your computer for creative projects. Making backup copies of your Cakewalk files is quick, easy, and can save you some serious headaches, as well as hard drive space.
The preferred method for backing up files containing audio data is to save them as Cakewalk Bundle (.BUN) files. Here's how to do it in Cakewalk:

1. Choose File | Save As.
2. Change the Save As Type to Cakewalk Bundle.
3. Name the file.
4. Choose your saving location, and save the file.

Why use Bundle (.BUN) files rather than Project (.WRK) files for backing up?
First, a quick lesson: A Project (.WRK) file contains MIDI data, project-specific settings, and references to audio data that resides in the location specified by the current Data Directory (Options | Audio | Advanced). Backing up the .WRK file, or transferring the .WRK file from one machine to another, will NOT automatically backup or transfer the audio clip files referenced by that .WRK file. So saving or copying a .WRK file alone is not enough to safeguard your projects or transfer them to a different computer. 
The .WRK format is perfect for everyday Cakewalk use, and we don't suggest you save as a Bundle every time you save. However, once you've come to a point in a project where you've made significant progress and feel the need to back up the file, the Cakewalk Bundle comes into play.
The Bundle (.BUN) format preserves ALL data, audio and otherwise, in one file, which can then be moved to any location you like. Because of this, these files also tend to be quite large and slow to open. You'll need to save your Bundle files on your hard drive, or on some large capacity storage medium such as a CD or tape drive. Once that's done, you can delete the .WRK file version, then use the Clean Audio Disk command to remove the associated audio files (which you don't need because they're saved in the .BUN file now). This will free up space on your hard drive for your next project.
When you're ready to work with the project again, simply choose File | Open and navigate to the location of the Bundle file. The file will open with all data preserved just as you left it.

looks like they need to update the storage media for large devices to include externaL HARD DRIVES OR THUMB DRIVES WHICH ARE SIGNIFICANTLY LARGER THAN cdS NOW...

stupid caps lock...

Bundle files have worked for me for well over a decade.  I save them in several places, to be safe, but I've had good luck with them being reliable.  I'm not sure why you think they're unreliable.  I hear this rumor a lot, but I've also heard that they work 100% of the time, too.  I know that sometimes people save them to a different hard drive which then goes south, but backing them up to CD or DVD is a good practice in addition to hard drives.  The point is: when backing up projects, use several locations and methods if you're paranoid.  Or, use Gobbler?

http://forum.cakewalk.com...ning-cwb-m3030558.aspx
 
http://forum.cakewalk.com/Cant-open-cwb-Error-File-is-corruptInvalid-audio-format-m1784350.aspx
 
 

During the past 8 years or so I think there's been at least one post a week about something wrong with bundles.
They are surely good as long as you don't depend on them. 

Bundles suck.
Before I started using Per Project I lost many projects due to corrupted ones.
I still have some that open but I wouldn't recommend using them to anyone.

I am with Lynn regarding bundle files.
OK, first of all, you will need like 2 different drives to make 2 different backups, just to be safe.
I usually make the first bundle backup. Then close Sonar, and reopen the bundle file, play it, and once everything is OK, then save it again as a bundle file on another usb drive.
Of course along with a regular cwp file.  Just to be sure.
Also, bundle files are great for getting rid of stuff that you don't want it, even after a Clean Audio Folder, which takes a long time by the way. 
If any one wants to test this, save any project that you have been working on and off as a bundle file, and look at the size of the bundle file, compared to the size off the audio folder. You will be surprised to see how Clean Audio Folder has not deleted all the stuff that you don't want.
Why I am using bundle files all the time, is the only way to bring the size down, and then after reopening the bundle file, save it again as a cwp file in another folder.
So give it a try before condemning it. If it really sucks like Carl had stated, then probably CW had got rid of it a long time ago.
dan
 
 

I have maybe transferred 10 bundles in the last two years.
 
Never a problem.
 

As others have said, I would never rely on a single back up method for anything that I deem important, but never had a problem with cwb files so far on the occasions I have used them.