You’re right. Surges happen and they can destroy drives. But personally, I leave my external drives plugged in all the time.
There are a few things to be aware of before doing that.
It’s all about the power source
The drive’s power source is perhaps the single biggest factor. If you plug the main 120- or 240-volt power into the external drive, then it actually has its own power supply inside. That puts it pretty close to the same risk as your desktop computer.
Most external drives don’t have a power supply. They usually have an external power “brick” that converts the main line power to something else that’s actually used by the drive. That process of converting that power acts as a nice buffer from many types of power problems.
Some have no power supply at all, drawing everything they need solely from the five volts provided by the USB interface.
In these cases, I’m actually not overly concerned about surges.
Something certainly could happen, but I put the chances at very low.
The “Should I leave it plugged in all the time?” issue is really most often related to malware. If your external drive is connected and malware infects your machine, that drive could also get infected. This has nothing to do with power and power spikes; it has everything to do with software.
If the external drive actually never moves, then it’s still not something I’d worry about beyond making sure that my system as a whole is secure. If that drive travels from machine to machine, then maybe it’s worth a little extra thought. But even then, I’d still focus on keeping the system clear as a whole – where the side effect is that the external drive never gets infected.
The good news is that backup images from programs like Macrium (if that’s what the drive is used for) are typically not compromised.