A few thoughts.
Firstly, as others have pointed out, three failed hard drives in a 10 year span is neither unusual nor unexpected. The rule is not to wonder IF your hard drive will fail, but rather WHEN.
In my view, the solution is not to seek out a setup in which a drive will never fail, because that is just not feasible, but rather a solution in which you can recover quickly and completely from a drive failure, and if at all possible, anticipate a failure before it happens as well.
Since you would prefer to store your data files on an external drive (not unreasonable; I actually do the same), it is important to consider that your 2009 iMac has, I think USB 2.0 as its fastest interface and so your data transfer speeds are going to be much slower than you would achieve using the internal drive. Still, since that works for you I am not suggesting you change it, but rather use a setup that achieves the above goals.
What I might suggest for your involves a number of things you can do; these are not mutually exclusive and provide a number of recovery options.
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Consider getting an external drive that actually contains two separate drives in a RAID1 configuration. Thereby the two drives are duplicates of each other, and as such, if one fails, you are still fully operational on the other until you buy a replacement and copy everything over.
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A second external drive of the same size would be used for a regular clone of the working drive. Software already mentioned includes SuperDuper!, Carbon Copy Cloner, and also ChronoSync, listed in order of ease of use (easy to hard) and capability/flexibiliy (least to most). I actually do a DAILY clone, so that if a drive crashes, my clone is at most 24 hours out of date. I personally use Carbon Copy Cloner but also used both others and all would work fine. I have CCC set up to save all files being removed (eg files deleted from the working drive that therefore should no longer be on the clone), and so I can also retrieve accidentally deleted files as well, and I would encourage doing this, but then you will need the clone drive to be bigger than the main drive to ensure enough space for these files. I periodically purge out these archival folders, every few months.
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Time machine is not a bad idea, which would be a third external drive, because while it is filled with bugs and unreliability, it often does wind up working, is a real-time backup, and therefore is worth having in place. I have restored a file or three from a TM backup over the years, and it’s quick and easy to do so.
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Finally, I totally agree with the other folks who have recommended that you have an offsite backup. If you care about your data, this is a must. Many people use BackBlaze which has the advantages of simplicity and low and fixed cost for unlimited data (from a single computer). As already noted, BackBlaze will backup data from directly connected external drives, provided they are connected at least once every 30 days. You would exclude the TimeMachine drive and the clone from your backup as being unnecessary and a waste of your network bandwidth.
My problem with BackBlaze is a simple one. Any file deleted from your computer (internal or external drive) will be removed from your BackBlaze store after 30 days. I have had a circumstance in the past in which I presumably accidentally deleted a large amount of data and did not realize it for about 6 months. How that happened I have no idea. With BackBlaze I would have been out of luck in data recovery. Since I use Arq instead, however, I was able to restore all of my data.
Arq, the program I use, is a more complex setup and does not come with storage; rather you point it at the online storage service of your choice (I use BackBlaze B2) and it does its thing. I have it set up to keep everything so I can go back in time much more than 30 days if necessary. Because all of my computers backup to the same B2 account, all of my data is in the same place and since I pay based on what I use, there is no issue of having more than one computer backing up on the same account. The cost is higher but worth the cost for me. The maker of Arq has also launched another produced called ArqBackup which provides a more “BackBlaze-like” setup that might be a better fit for you. Here you play for storage in 1TB allocations, and if you have several TB to store it will be much more expensive than BackBlaze, but because you pay more for more storage, you can save files indefinitely after deletion if you prefer. I have not tried this product as it would not be economical for me as I back up 4 Macs and by lumping all in to one store, I am not paying one fee for each computer which doe snot fully fill the pre-specified storage limit. I would encourage you to adopt one of these offline backup solutions.
Finally, people always ask what brand of drive to use, and frankly any and every drive manufacturer will have failures. People often point to BackBlaze’s yearly summary of device failure rates to use in vendor selection, but every vendor has had models that are more or less reliable than others, so there is a lot of variability that I think makes it hard to prefer once vendor over another. Seagate and Western Digital are the “big boys’” in this market and I think both make fine products. Both have a range of drives with some specifically geared toward “consumer” low burden use, and others geared towards the high end server continuous operation market. Some like to use only one brand feeling they are “the best” and others like to use different brands for their working drive vs their backup to hedge bets against a bad model. I use WD Reds (server grade) in my Drobo working drive, and a mix of WD Reds and Seagates in my backup Drobo, and while I have had good luck with both brands, I believe the correct solution is again to ensure adequate backup and recovery protocols are in place rather than to try to find a drive solution that will never fail.
Lastly: whatever you set up, make sure it is a solution that happens automatically, eg set up your clones to happen automatically every day/week or whatever schedule you want. If you rely on manually triggering clones you will forget or get busy or whatever, and no strategy can help you if you fail to use it.
Hope this is helpful.