Do all external hard drives work with all Operating Systems? Since they External HDDs are hardware (and NOT software), I imagine that it doesn't matter if one connects an External HDD to Windows, Mac, Linux etc.? Right?
I'm no longer so sure because I often see External HDDs advertised as being compatible with Windows.
An external hard drive should work with any operating system as long as it has a connector that's compatible with the computer. For example an external hard drive that only has a USB connector can only be used with a computer that has a place to plug in a USB device (USB is super-common, so it's very unlikely this would be an issue).
However some computers store data on the drive using different file systems. For example the standard file system used by Apple is different than the one used by Windows. This means if you want to write data onto a hard drive using an Apple and then plug it into a Windows computer to read it, you're going to run into some issues.
But if you just plan to use the hard drive with one type of OS then you simply have to format it with the file system appropriate for that OS and then it's clear sailing.