Why is my hard drive suddenly read-only? Only ever used on Mac.

oswaltk wrote:

Ok, so why would this happen? And what do I do now?

The reason is that hard disk drives don't last forever. The solution is to replace it with a new one.

macOS will make an effort to preserve files that may remain intact by changing its status to "read-only", thereby preventing further alteration and loss of data. Since the drive has suffered a failure it should no longer be relied upon to function.

How do people save files?

I can't speak for what others do because only they can place a value on their data. In my opinion essential data needs to be backed up. Only one backup is not sufficient because any drive can fail at any time. Failures can arise gradually, with some warning, or catastrophically, with none.

Apple has provided a backup solution for well over a decade. You already have it and it's free. To learn how to use Time Machine please read How to use Time Machine to back up or restore your Mac - Apple Support. It will back up all locally connected volumes to as many backup devices as you wish.

Do you save thousands of pictures and files on MORE than one device?

Certainly. To have just one of anything essential is tempting fate.

The disk’s format appears to have been changed to MS-DOS. When that occurs spontaneously it means the disk drive has failed.

Drives can fail at any time. Assuming you can in fact see/open the files on the drive, you would have to purchase a new drive and copy the files over.