"UMDs are 2.4-inch optical discs in protective cases that have a data storage capacity of 1.8GB."
GB means gigaBYTES not gigabits, sorry.
Found more info
# Media:
# UMD--Universal Media Disc, 60 mm diameter
# 660 nm laser diode
# 1.8 GB dual layer
# 11Mbps transfer rate
# Shock-proof
# Secure...