Every now and then you get a problem with OS X’s Time Machine function. When that happens the system asks you to start a new backup which results in you losing all previous backups.
It hasn’t happened to me for more than two years or so and even if it does usually I have something in place so that I don’t lose my backups: My NAS backs up to another NAS that can take snapshots, so usually I’d be able to rollback to a previous snapshot of the Time Machine.
Unfortunately, when this happened to me recently I had just deleted the previous snapshots because I was running out of space and didn’t want to buy another hard disk – so I couldn’t roll back to a previous snapshot.
Well, for now I have just created a new Time Machine share, meaning that if I need to go back I can access the old Time Machine in the previous share.
PS: When I had an Apple Time Capsule this problem occurred every few months. With y Synology NAS it only seems to happen every few years.