- Physical risks-not subject to the same physical risks as the original data, e.g., stored well away from the source computer.
- Different hardware-a backup stored on the same drive as the original is not a backup. Multiple backups on the same drive of the same data are onebackup; they all are lost together; they are only semi-redundant.
- Redundancy-in the event that a backup is needed, a single backup will not do; it becomes an original. Hence a bare minimum of two full backups is essential (three or more preferred on separate drives).
- Data integrity-a backup might be initially fine, but that is not guaranteed to last over time. A program like IntegrityChecker for down-the-road data validation is important in some situations, including DVD and BluRay backups: how can it be known that the files remain intact?
- Eliminate potential for user errors: use cloning and establish naming conventions.
- Eliminate potential for hardware confusion: mixing many too-small and old drives for partial this and partial that is a serious risk (from complexity). Migrate to large external hard drives with a long lifespan (enough capacity).
- Redundancy can include mixed storage types, e.g., hard drive backup, DVD or BluRay disks, cloud storage.
- Hard drives do not last forever and should be replaced every 3-4 years. They should also be erased at least once a year, and a fresh backup made (to re-magnetize and to force use of the drive on which data is stored).
- Always perform manual “sanity checks”: do not assume the backup process has worked. At the least, use the Finder Get Info window on the original and the backup.
#1 My system uses a RAID-1 mirror or RAID-5, so my data is safe.
RAID mirroring and RAID-5 is fault tolerance. It is not a backup.
If the machine is stolen, the RAID is likely to disappear along with it. If there is a fire, flood, massive power surge (lightning), the RAID is likely to be lost also.
#1a My backup drives are RAID so I only need one backup.
This is a grave mistake. Loss or damage to a RAID and it’s gone too. A RAID-0 stripe is even more at risk from failure of any drive in the RAID.
RAID is not only more expensive, it offers less redundancy for the same money, and thus offers little or no value for backup. See Redundancy in the introduction above.
For example, a RAID-1 mirror uses two drives with one power supply, which can also fail, possibly frying both drives. Far better to have two drives each with its own power supply and each thus storable separately, to mitigate physical risks. Three is even better and probably costs no more than the single RAID mirror unit.
#2 Time Machine works great for me .
But Time Machine is slow to restore, not directly accessible, has some bugs, is not bootable, tends to fill up with unwanted versions, and in most cases is located physically close to the originals on the computer (e.g. directly attached). See notes on RAID above.
#3 Copying files works OK for me.
Congratulations on perfect memory of what has changed, including deep knowledge of where everything lives on the system, as well as your charmed-life avoidance of various silent Finder-copy bugs.
#4 No problem, I have a backup connected all the time.
See notes on physical risks.
#5 I have backups on other drives
Are these backups or originals?
A critical question to which the answer is sometimes “originals”, based on your author’s consulting work.
#6 I have backups on 20 drives, but I’m not sure which ones are which.
A backup system which is confusing or hard to remember is a serious risk. The “savings” of thirteen mixed-size and capacity drives each with partial this and partial that increases the risks of data loss because it becomes difficult to remember what is where and when. Move to large capacity backup drives and cloning.
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