My machine keeps getting ejected when computer is in sleep mode.

For the past several months, when I put my Mac to sleep, my Time Machine constantly keeps getting ejected and it says "Disk not ejected properly" and "Eject time machine before disconnecting or turning it off. " When I wake my Mac, the Time Machine doesn’t even show up on my desktop, and it is grayed out and in disk utilities. Does anyone know how to resolve this issue? I don't want the time machine to be disconnected whenever I put my computer to sleep. Also, I have replaced my portable hard drive with a new one because I thought it could have been the problem with the drive, but that doesn't seem to be the problem.