Powder Mill has been around for a long time. A long time ago it was probably even a cutting edge trail. These days it needs some love. That love is in the works. I gave it a two-star quality rating, but with some work this will be at least a four-star trail.
Park at the trailhead off of FR 9a. Ride left along the creek until you have no choice but to cross (within sight of your car). Ford the creek and find a rugged, steep old logging road going up the hill. Hike-a-bike up this lower section until you escape the mud and can ride (a few dozen yards) and reap the rewards of your persistence.
The lower and upper ends of the trail have short unrideable sections. But the rest of this trail is amazing, scenic, and enjoyable. Once the trail gains a natural bench after the initial climb up from Indian Creek, it contours for a long time over a deep, narrow holler (Powder Mill Branch) and crosses a couple of smaller drainages coming in from the right (north). A short side hike that is worthwhile at the first drainage will take you to a rare limestone arch.
Once back on the trail, continue the steady climb up Powder Mill Branch through a beautiful boulder garden, and then an intimate little valley before confronting the upper wall exit of the trail. There is a short, mandatory, hike-a-bike and the,n if you're strong enough, you might be able to ride on out. Mere mortals may have to keep hiking.
At the top end, you gain Hatton Ridge Road
which is a relatively unknown Forest Service Road. You can ride along the road and pick up another obscure gravel/dirt road and circumnavigate the entire Spaas Creek and Short Creek rims before returning and reversing the Powder Mill Trail back to the beginning.