Time Machine Disk

I formatted a 2TB external hard drive using Disk Utility and then immediately started using that hard drive for Time Machine backups. The first backup finished just fine, but when I tried to put another file onto the hard drive, it won't copy to it.

The backup is there and there's lots of room on the disk for other things, but it won't allow me to copy any to that disk.

I have a 4TB hard drive which I also use as a backup disk, and I am able to use it "normally" as well.

I am confused.