Are SATA hardrives interchangable?

Yes, they are.

Here's the thing though. You can't switch drives and expect the computer to boot what's in it, outright.

There are ways to make it work. Research on it.

It should be able to boot you might have to go into bios and select that drive to boot from.

The only thing that would be a problem is that it would load the drivers of the old computer and that is where you might run into problems 

What your trying to do will not work.   When you install an operating system, drivers have to also be installed.  When you insert the HD with Windows 8, it will not boot properly However.....You will be able to look and copy any files.

win 10 will not allow a second bootable drive [like the one from your old computer] to exist in the system without some fancy footwork. pls search for how to fix this issue in techie places [not here]

There are a lot of dependencies, you probably will have issues.

1) Is the SSD in your laptop an M.2 stick or a 2.5" drive? If it is a 2.5" drive, why would you replace a high speed SSD with an old unreliable slow HDD? If M.2, you cannot replace it, but there still could be a place to add it.

2) The drivers are different. It will have issues in booting. If it can be swapped, the question is about the 8.1 license. The laptop could be a Win 7 upgrade to 8.1, or an original 8.0 or 8.1. If a branded PC that came with 8.1, the license is in the bios of an IC on the old laptop motherboard and Microsoft won't allow a drivers update. If it was an old Windows 7 originally factory installed, that brand won't support you. Windows 10 is transferable.

So, if it physically fits and transfers as hardware, most likely you end up with the exact same operating system and software you have now anyway, after a large effort.

If you want to use the space for extra storage, buy a 2.5" SATA USB Enclosure for the HDD and use it as an external drive. You can erase what you don't need. You'll have issues using some of its current software one way or the other.

Can you state your newer laptop by brand and full model and what you are trying to accomplish?

That, will work fine!! Just make sure they are the same form factor. ie: 3.5 or 5.5

As long as the connectors are identical, you should have few problems.  Note, however, that the old Windows 8 install may not have the drivers necessary to run the equipment on the new laptop.  When you boot up either, in either machine, you will see it boot slowly the first time.  This is windows locating and installing drivers for the laptop it is in.  The Win 8 HDD will boot more slowly as it endeavors to find drivers.  If it boots, you may want to check out  the device manager to see which items do not have drivers and begin looking for them on the net.  

Pull the hard drives out and look at the connecting power/data pins at the end of each,if they match exactly then you can interchange them. Be careful when taking them out,they slide out,don't lift. Don't force them back in. If you damage any connection either on the hard drive or the PC it's finished.

Yes and no.

Yes - quite often - if the laptops are the same manufacturer and not too many years apart.

No if the laptops are different companies.  See, when you power up a computer it's pretty stupid.  The first injection of 'smarts' is BIOS - basic input output system - after BIOS the computer learn to grab 'driver's off the hard drive - this tells the computer how to deal with all the features on the motherboard and things plugged into it - from mouse and keyboard to wifi.  You pop in a different hard drive, say from a Dell latitude to a newer one it will probably go.  Pop the newer drive into the older one and it could go do.

Pop company A drive into a computer made by company B - the drivers wont be there, however, the computer will try to make it work by loading Windows generic drivers.  Here the laptop will start and work - however you wont get the screen resolution until that driver is loaded, touch screen might not work eieter…. however the keyboard and mouse should.  I would expect the LAN would work but it 50-50 for the wifi to load.  If you just want to get some old files off - you will be fine.

I have had to swap drives allot at work when I first got here.  I've never damaged a computer or drive doing it.  You want to wear a static strap on your wrist and make sure you work on a grounded table...…..static kills hard drives!!!!!!!!!!!  You should read about proper "ESD Procedures" online before doing this.

It isn't that simple, you need the drivers for the hardware to be on the drive to efficiently interact the OS to the Hardware.