How do I prevent my disk from needing to format and erasing my files?

I simply remove the disk from the camera and use a card reader, sometimes I take it out the camera when it's on and running or I'll forget to take it out of the card reader or camera.

But I've formatted it twice now, switched cards and don't know why this is happening?

I have a 2015 mirrorless Nikon

Don't use it

Read the camera manual. It says to connect the camera's USB port to the computer's USB port with a USB cable and put the camera into computer mode.

Since you did not tell us what computer or operating system, we don't know why you keep having the problems.

When you follow the manual, then you can get support from the camera manufacturer. When you don't and don't tell us complete information, we can only point you back to the manual.

I transfer video and images both ways (connecting the camera and using a flash memory card reader) and don't have problems… I use a Mac that is running the current macOS Sierra (10.12.5)… Or Windows 10 on an HP laptop…

NEVER remove your memory card from your camera while the camera is turned on.

That is the first step.

Next develop this workflow as a habit and you will no longer have any problems like the ones you have alluded to in your question

* At the end of each shooting day, remove the memory card from your camera/
* Insert it into your card reader
* Using your computer skills, copy (do not move) all the image files to a new file folder you have named for the subject/location and date on your hard drive
* Once all the images are safely on your computer, reinsert the memory card into your camera.
* Use the format feature on your camera to format the card so it is ready for your next photographic adventure

Do this routinely and you should not have any more problems.

NOTE: It is always a good idea to backup all your original RAW files to either an external hard drive, NAS, burned DVD or BD disc and perhaps use a cloud backup service like BackBlaze. Websites like Photobucket, Flickr, DeviantArt or other photography "sharing" sites should ever be used to store your images.