It’s an interesting and valid question. You should be concerned about computer crashes, but I suspect that your question actually includes a flawed assumption.
When your computer crashes, the only thing that you’re guaranteed to lose is what’s in memory. That’s the RAM – what’s “erased” when you turn the computer off and what gets filled when the computer boots, you run a program, or you load data to work in a program.
A crash can affect your hard disk, but it’s actually rare to lose all of the data and files. The same is true for external drives, but it’s even less common that they would suffer some kind of data loss. It can happen, but it’s rarer than your question might imply.
How a crash happens
Ultimately, the nature of the crash, the disk activity, and the files being written at the time of the crash are what matter. If your computer was writing data when a crash happened, it’s that data that would be at risk of being lost or damaged. Yes, if that happens to be critically important file system data it could impact a lot of what’s stored on the hard drive, but more commonly it’s just whatever part of whatever file it was writing at the time that gets hurt.
If the software crashes, there’s no physical damage to your hard disk or the drives. Any damage you may see is typically just data, and data that’s isolated to one specific piece of hardware, such as the device that was being written to at the time of the crash.
Of course, all bets are off if the crash is hardware related.
When hardware crashes
If the power supply has a problem or your machine gets hit by lightning, the electrical surge could cause problems on multiple devices simultaneously. Again, it’s not very common, but it sometimes does happen.
That’s why I talk about things like surge protectors or uninterruptable power supplies, particularly if you’re in a location where power problems are common.
You need to protect yourself from things that can cause unexpected hardware problems on your computer.
But even then, having everything get destroyed at once is incredibly rare. In fact I’d say the most common cause for that type of catastrophic problem has nothing at all to do with the computer, and more to do with the house it’s kept in burning to the ground.
But, of course, that’s also why you’re backing up regularly.