It is caused by something other than a RAM disk I am afraid. Below is a screenshot from my computer where you can see that even having a 4 GB RAM disk in place does not reduce usable memory. The fact that the volume of unusable memory on your computer is similar to the size of the RAM disk you used to have, is just a coincidence.
If Windows shows some memory as unusable, it is usually reserved by the system for some hardware. The most common example is an integrated video card that uses some of the RAM for itself.