Dell OptiPlex 990 - Drive Bays, Connectors, and Controls

Cards
Mini-Tower up to one full-height card
Desktop up to one low-profile card
Small Form Factor none
Ultra Small Form Factor none
PCI Express x1
Mini-Tower up to three full-height cards
Desktop up to three low-profile cards
Small Form Factor up to two low-profile cards
Ultra Small Form Factor none
PCI-Express x16
Mini-Tower up to two full-height cards
Desktop up to two low-profile cards
Small Form Factor up to two low-profile cards
Ultra Small Form Factor none
Mini PCI Express
Mini-Tower none
Desktop none
Small Form Factor none
Ultra Small Form Factor up to one half-height card
Drives
Externally Accessible (5.25–inch drive bays)
Mini-Tower
two
Desktop
one
Small Form Factor
one slim optical drive bay
Ultra Small Form Factor
one slim optical drive bay
Internally Accessible:
85