Do you have more than one backup of your data, such as two external hard drives? Just one? More than two?
First decide if you need the data. If you do then make sure you always follow these two rules, even while a backup is in progress:
1) The data needs to be on three different physical media at all times. So if you backup system over-writes the old data, you will need four copies of the data. This is one reason way incremental backup is best, no data is over written until a newer version is written. Time Machine and a few others are incremental. Two copies not not enough
2) The data need to be always in at least two geographic locations. Even during a backup. So you would need to either rotate disk to the office, always having one there or use a cloud time off site backup service
So three copies of the data, the live data and two backups is the MINIMUM. Here is a reasonable system
Buy an external disk for Time Machine. This disk needs to be about 1.5 ties larger than all the data you have. The external TM disk will likely be the newest and latest disk you own. Let it run 24x7. If the computer is a notebook then maybe connect that to the airport router. But TM is your first line backup and it runs every hour.
Next make a copy of the data using whatever software you like and keep the copy in another room onside a fire safe. When I buy a new Time Machine disk I use the old TM disk for this. It MUST be unplugged and in a good container.
Then I subscribe to CrashPlan and let them keep a copy of my data. It takes weeks to move it all but you only do it once. Another service I like is Backblaze. The latter is MUCH simpler to set up but less flexible. Backblaze is also much faster.
That is the minimum. If you don't like clouds then buy one more disk and one mrs fire safe and keep it at where you work or some other far away place and rotate it with the copy you keep at home.
What are you worried about? Not a failed disk. That is easy to recover from if you have just one TM backup. You should worry about:
1) Theft of the equipment. This is a common cause of los data. The guy will take your backup drives and anything else he can cary
2) fire, flood, earthquake. Yes it isunlikley but everyone who lost data in a fire said a fire is unlikely. It will of course destroy the backup drives in your house.
3) Lightening hits a utility pole down the street and 2000 volt surge kills every computer device plugged into AC. Goodthing you have a backup in the safe.
4) Screw-ups by the operator, the OS or the software. This is the #1 cause of lost data.
One more thing buy a new disk every year or so. Get the largest size they make, currently that is 4TB. Use that for Time Machine. Then use the old TM disk for the copy you keep in the safes and retire your oldest disk. This way none of your backup are written to drives that are more than about 4 years old. The cost is only about $120 or $150 per year. Yes the backup drives must be larger because incremental backup use spec to keep older versions. Also it is very important that the disk drives be used, writing new backups to them is a kind of test. They WILL FAIL and you want to know when that happens, so you rotate them.
Way be so carful? If you want a photo to last for decades with 99% certainty you need more than the minimum.