I have a health issue and my unit is going to give me a medboard. Does this mean I am being separated?

The IDES system almost by design is slow and you have a lot of due process. I have seen people retained, but most are separated. The first step is the doctor issuing a non-deployable profile (P3). Without that profile the unit can want a med board all day. Regardless of medical condition or personal preference you need to do everything you can to maximize your fitness for your personal best interest. As a reservist(would be the same for active) you need to start working the next phase of your life regardless. Good luck and if you enter the IDES you'll get a full interagency briefing on the process and your rights.

As with any board, this is merely an evaluation of your current condition. Being a medical board, they will evaluate your current level of health and decide if you're able to be retained in the Army and that your medical condition won't affect you performing your job. Medical boards do not automatically spell separation.

You can appeal the board's decision. Their decision is not final. You should be able to actually meet a board after the local board makes a decision. Don't take NO unless they at least attempt to appease you.
Good luck.