Let me say this first: Backups should be external to the Mac. A backup that is internal to the Mac, even if it is on a second drive you have installed, will be vulnerable to the threats to the internal drive it is backing up. For obvious reasons, a backup on the internal boot drive is not really a backup at all because if the internal boot drive fails, the backup is lost along with the originals. That same risk also applies to different partitions or different Containers or Volumes. If the backup is on the same hardware, it is at risk of that hardware failing and taking with it both the source and backup and you lose your data, forever. So, backups should be external to the source.
Planning backups and where to put them is a little like buying life insurance. You have to think about unpleasant things to make decisions. But if you don’t make those decisions and something DOES happen, you will be much more uncomfortable than the planning process made you. And that fact is never truer than planning WHERE to store your backup drive(s). To plan, you need to think about what might happen, what disaster might befall your system, to have a backup that is robust enough to survive and let you get going again.
OK, what media to use? The range and quality of external storage varies widely. One option is to store the backup on a flash drive, thumb drive, or memory stick or storage card. Those type of devices come in large sizes these days. I did a quick search of Amazon and found a 2TB flash drive! It wasn’t even very expensive. One could make backups to that flash drive, it’s certainly large enough. However, the downside to those devices is that they are not really designed for long term storage. I’ve had more of them fail than any other kind of media, so I don’t use them for anything I don’t want to lose. They are great for transporting a lot of data in a small format, but not really very good for long term storage.
A “real” drive has more redundancy and generally has a longer expected life. I prefer them for backups. You can use either rotating drives, which are less expensive, or you can use SSD drives, which are faster. Frankly, after the first backup, if what you do are incremental backups, i.e., you only add to the backup files which have changed, the speed of the backup drive is not all that critical. That is, unless you are trying to make a bootable backup. In that case it may be worth getting a more expensive SSD drive for the backup. I also use network attached storage (NAS) for a backup destination, as well as a RAID array.
RAID (Redundant Array of Inexpensive Disks) is a technology that adds another hardware layer of reliability to storage. At the lower levels of RAID, the system is simply two disks that are mirrored (or duplicated) by either the RAID software or hardware. When you write to a RAID array, the system sees it as one disk, so it writes the data just once. Internal to the array, the data is written first to one disk, then the RAID firmware immediately copies that data from the first disk to the second. That way, if either disk fails, you can pull it out, put in a new one and the array will then mirror from the surviving disk to the new one, all pretty much invisible to the user. One of my backups goes to a RAID array, so it, in effect, makes two copies of the data. RAID can get more complex, if the value of the data warrants it, to the point where you can hot-swap a disk and never see any effect on the system! But a discussion of RAID technology is beyond the scope of this post. I’ll leave that for your own research. But remember this, a RAID array is NOT a backup, just a more secure location to store files.
The disaster that befalls you might not be just a computer issue. It could be your house being hit by a meteor, or aliens blowing up your house for some strange and unknown reason. Or it could be as mundane as a broken water pipe spews water into your home office for two hours before anybody notices. In those kinds of disasters, having a backup stored somewhere else could be a very good thing. A backup right next to your system is handy, but if the disaster is wider than your system (as a lot of them are), it could wipe out the nearby backup drives as well.
Ways to store backups off-site range from storing a backup drive in a bank safety deposit box, sending one to a friend to hold for you, giving one to a trusted neighbor, or maybe even making a backup into an internet backup provider who will store your data in a storage location far from your area. The advantages of off-site storage are that you have protection from any physical damage to your system that is more widespread than just the system itself. The downside to off-site storage is that it is a pain to maintain. The closer the backup is to your location, the faster you can get it back. But then it is at greater risk of a regional disaster (hurricane, blizzard, earthquake, tsunami, etc). The further away it is from your office, those regional impacts are much less of an impact. But then the sending/receiving time for the backup starts to get long and expensive. Online storage of backups is dependent on Internet access, a service which may or may not be available to you if there is a regional impact. Again, like all other aspects of backups, the issue is that you need to make a determination of what is important to YOU in YOUR situation.