I use Mozy for my offline backups and I am very pleased with the service. I moved to Mozy because my previous service did not support external hard drives.
I did run into a situation with my Mozy backups that should have been immediately obvious, but I did not catch it for a while.
The first backup you do with an offline service takes a good while. I backup about 30G which is certainly not an exorbitant amount, and the first backup took 3 – 4 days elapsed time with the computer not running 24/7.
Each day the backups run anywhere from 40 minutes to an hour. However, ever now and again the backups seemed to start over, and take the entire 3 – 4 days again. It happened again this week so I finally stopped to figure it out.
Every now and again I would start the computer without the external hard drive – which I do from time to time if all that I am going to do is use the Internet. The problem happened when the automatic backups started during the time the external hard drive was off. Mozy did the little bit of backups from drive C: and then assumed 29G was deleted, and deleted it from my backups.
The next backup had to start over. Once I figured it out, it was easy enough to control, but it took me a couple minutes of thinking to figure out what was happening.
A quick email to Mozy support (which is very responsive) confirmed my diagnosis.