Many people make sequenced back ups, i.e. have a number of back ups made on different days in case one of the back ups fails as well as the master.
One of the most common schemes is to have three copies "Grandfather, father, Son (GFS)".
Say you have three external hard-drives HD1, HD2, HD3
Week 1 : backup to HD1 = "Son"
Week 2 : backup to HD2 = "Son", HD1 = "Father"
Week 2 : backup to HD3 = "Son", HD2 = "Father", HD1 = "Grandfather"
Week 4 : backup to HD1 = "Son", HD3 = "Father", HD2 = "Grandfather"
Week 5 : backup to HD2 = "Son", HD1 = "Father", HD3 = "GrandFather"
Week 6 : backup to HD3 = "Son", HD2 = "Father", HD1 = "GrandFather"
Week 7 : backup to HD1 = "Son", HD3 = "Father", HD2 = "GrandFather"
Week 8 : backup to HD2 = "Son", HD1 = "Father", HD3 = "GrandFather"
Week 9 : backup to HD3 = "Son", HD2 = "Father", HD1 = "GrandFather"
etc.
Businesses might have 7 such copies (and backup daily) with extra backups per month.
It depends on how important it is to have copies to come back to, and - especially for us - how much time it takes to replace all the files, images, and information you have collected in the course of time ( if indeed you can actually find replacements !!)
more on rotation schemes here:
https://en.wikipedia.org/wiki/Backup_rotation_schemeIn general I try to follow the GFS method (roughly once a month) and on days when I am doing a lot on the computer (editing files, etc.), then I also make backups (almost) immediately after.
Bob
ps. I have mentioned elsewhere, that some people use the OSISHMAB method.
i.e. in the moment when the hard-disk crashes (irretrievably) they say:
Oh
S***
I Should
Have
Made
A Backup
Speaking as one who knows this method (very well !!) I can only say, regardless of the method -
Make Backups - Often !!!