Quantum DLT 2000 Handbook page 134

Magneto-optical (MO) drives use a combination laser/magnetic
head to achieve higher storage capacities than PD drives. However,
higher capacity comes at the expense of compatibility with stan-
dard CD drives. MO disks can only be read by other MO drives.
The highest capacity MO drive on the market today offers 4.6GB
of storage, hardly sufficient for serious storage applications. MO
drives are slow during write operations because they must erase
each disk sector before new data can be written.
In general, optical media is prohibitively expensive for large vol-
ume backup and archiving applications. In addition, optical's low
data density means it would take numerous disks to equal one
DLTtape cartridge. This extra administrative burden adds to labor
costs and logistical headaches.
Next, we'll evaluate the obvious and hidden costs of owning and oper-
ating various data backup systems, including the DLTtape system.
11.12 DLTtape SYSTEM TECHNOLOGY COMPARED