Rotation Speed = 3000 rpm = 50rps, 1round = $\frac{1}{50}$ seconds
Track Capacity = $512*1024 = 2^{19} =512KB$
Bytes to be transferred = 4B
Transfer Time = $\frac{4}{50*512*1000}$ = 156.25ns =4 memory cycles
So, One out of every 4 cycles will be stolen = that is 25% (CPU % Block)
It means CPU will only be blocked for 1 cycle, and for remaining 3 cycles it will be busy doing its own work!