Before
Add() called, but no partial cluster in category 1
Category 1
Cluster A
128 / 128
Cluster D
128 / 128
all full, no head with space
Free Pool
F
G
H
F popped from free pool, linked at head of category 1
After
Category 1
Cluster F
1 / 128
head (new)
Cluster A
128 / 128
Cluster D
128 / 128
tail
Free Pool
G
H