Step 1: Count the recording surfaces.
There are $2$ circular disks, and each disk records on $2$ sides, so the total is $2\times 2=4$ surfaces.
Step 2: Recall how heads are mounted.
A disk drive has one read/write head per surface, and all the heads move together on a single arm.
Step 3: Define a cylinder.
The set of tracks that lie at the same radius, one on each surface, lined up vertically, forms a cylinder.
Step 4: Link cylinder to no head movement.
Because the heads share one arm, all tracks of a single cylinder can be reached without moving the arm at all.
Step 5: Count tracks per cylinder here.
With $4$ surfaces, one cylinder is made of $4$ aligned tracks.
Step 6: Answer the question.
So without moving, the reader can access one full cylinder, that is $4$ tracks.
\[ \boxed{\text{cylinder (or 4 tracks)}} \]