My backup strategy is simple. I have two computers, in two different locations, synchronized in real time over the web using Dropbox. Each computer is backed up hourly using Time Machine on two separate external hard drives. Time Machine keeps hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months.
- If I mistakenly delete or overwrite a file on either computer I have thirty days worth of revisions on Dropbox, and twenty four hours worth of hourly revisions on both Time Machine backup.
- If Dropbox fails to synchronize or inadvertently deletes files on both computers and the web, I still have two Time Machine backups in two different locations.
- If my house burns down I may lose a computer and a Time Machine backup, but I will still have all of my files backed up to the web via Dropbox, and my second computer with a working Time Machine backup.
- If the city of Boston burns down I may may lose both of my computers and both of my Time Machine backups, but I will still have all of my files backed up to the web via Dropbox.
Keeping an updated clone of either computer's hard disks is not important to me. If either computer fails, I have a second computer. I keep my software needs simple so I can restore either computer from backup in under two hours.
Maintaining my data on a RAID (Redundant Array of Independent Disks) is not the same as having a backup. When files are deleted from first drive n a RAID they are deleted from the second drive. There is no such things as revisions in RAID, and it is not uncommon for the RAID controller to fail and data to be lost on both hard drives. If a RAID protects you from anything it is a single hard drive failure.
If I wanted to strengthen by backup strategy I might include a separate off site backup made at regular intervals independent of Dropbox. I might perform my Time Machine backups on a more redundant backup media like a RAID. No backup strategy is failure proof, but having your data in three locations, on two different mediums, and at least one offsite location is the goal. This is called 3-2-1 backup, and it is how I backup my data.