I have a few questions regarding this message.
First and foremost, why did I get this message?
Will I lose all of my backups for my MacBook Pro if i select Start new Backup?
Will I lose all of my backups for my iMac (which is also backed up to my Time Capsule)?
Yes, you will lose all the backups.. it is corrupted and has to be deleted and start over.
You will only lose the backups for the computer with the error message, so your MBP.. but you have no choice but to do it.
Why? Not known.. it happens.
Your deletion of backups from the imac will not affect the sparsebundle of the laptop.
If you use wireless .. that would be my suspicion.. wireless does not provide the data integrity you will get via ethernet. For the fresh backup make sure it is over ethernet at least for the first one.
If you want to keep a long term backup, TM is not the right tool.. use a disk image.. the disk utility can do it or use superduper .. even the free download can do a disk image.
Can you explain a little more about what you mean by "if you want to keep a long term back up, TM is not the right tool...use a disk image..."
Due to the fact that TM is an extremely complex type of backup.. read the details... in Pondini reference.. sorry our ISP has gone stupid.
A disk image you create onto TC or external drive is not altered or changed by hourly backups.. it basically cannot get corrupted. I really don't like the way TM handles incrementals .. that if one goes wrong it smashes not just the incremental itself but the entire backup history. It is an excellent tool for incremental backups .. don't get me wrong.. but eventually something goes wrong. It is in the nature of all devices to eventually err.. to err catastrophically is poor.
Also, how often does this happen? Is it rare?
It depends on the environment and how you work.. if you work via wireless on an upgraded OS it is far more likely to happen than over ethernet from a clean install. But happen it will, eventually.
BTW looking at your first post. The name of the TC is extremely long.. I think there is a limit to 27 characters. (I am not sure.. but for Mountain Lion there is already too much that can go wrong). I would shorten the names.. and in fact use SMB type.. short (max 20 characters ) no spaces, pure alphanumeric.
Help Mountain Lion remember where it is.. !! It has trouble with long names.
This is the reference now that my ISP has routing back up.. and why TM is rather more complex than it needs to be for long term stability.
Too much magic. http://pondini.org/TM/Works.html