The terms hard drive and hard disk have been interchangeable for as long as I have been using computers. I have preferred the term hard drive because floppy drive refers to the mechanical device not the media. To me hard disk more accurately describes the platters and not the complete assembly. But that is a personal preference.
Yes, the safest option is putting separate operating systems on separate physical drives, disks, SSDs, or NVMEs or whatever your preference for calling them is. I have successfully added numerous operating systems to the same physical drives by using numerous partitions. Windows creates 3 or 4 partitions when it is installed, and most Linux builds create 2 to 4 partitions when they are installed usually the primary partition is EXT4. This is why things can get complicated when you are installing multiple operating systems on the same physical drive.
The other thing that you can do is create bootable “to-go” disks preferably on NVME drives in a USB3 or higher enclosure. There are numerous ways of doing this with both Linux and Windows. You can even choose to have this operating system not recognize the other drives installed in your computer. This avoids the complications that can arise when you are installing multiple operating systems on the same physical drive. You just set your BIOS or UEFI to look at your USB port before it looks at the primary hard drive or in most cases today an NVME. When you have an NVME in a USB3 enclosure the decrease in performance is barely noticeable during most operations.
“The terms hard drive and hard disk have been interchangeable for as long as I have been using computers.”
Yes and now that we have solid state drives most just use “Drive” because there is no disk. Unfortunately it becomes confusing when discussing logical drives or physical drives. I suppose we should differentiate and identify them correctly as different entities.
The 3rd drive (used as an OS backup, programs, and redundant data drive) in the old Win 10 Pro machine (Dell 3420) is a 500 GB NVME drive in the M.2 slot -- which slot apparently was not intended for a drive, just network stuff, but will function with a drive. I could never get the BIOS to "permanently" set it as the boot drive, though: If I powered down and then rebooted, the machine would revert back to the SATA SSD. I could shut down and reboot yet again, and go into BIOS to boot off the NVME, but I never could get the darn thing to "remember" to boot off the NVME automatically. So, if a Windows update ran when I wasn't watching, the restart would bump me back over to the SATA drive (and likely an update there, too.)
For a little while I actually had a funny graphic as my desktop image, when the Win 10 Pro machine would flip back to the SATA drive: The graphic "shouted":
"Warning: Your DRIVING is all wrong!!"
But, I decided the SATA drive was plenty fast anyway, so, I ended up using the NVME as noted above.
Once I figure out what to do with the old Win 10 Pro machine, which means I'll almost certainly not need the NVME drive in it any more, that drive will be freed up for possibilities such as you describe.
IF I can ever get to all this stuff!
Tomorrow I'm going to be on the lookout for a deal on a modest capability Win 11 Pro desktop for my wife, as her teaching job at a local parochial school requires many, many hours of computer work at home, mostly on Libre Office, and she needs something automatically that stays up to date on security. Then her present computer is likely (again if I can find time!) to end up with Linux on it.
Thanks for all the info. / assistance!