Hard rives in optiplex 990

According to the specs for the 990:

Internally Accessible:

3.5–inch SATA drive bays
Mini-Tower two
Desktop one
Small Form Factor one
Ultra Small Form Factor none

2.5–inch SATA drive bays
Mini-Tower two
Desktop one
Small Form Factor one
Ultra Small Form Factor one

And if you pull the ODD then one more. But typically the ODD SATA port is not as fast as the HDD/SSD SATA ports. The USB connected drives would also not be as fast as an HDD/SSD port (SATA 3).

JOcean is spot on. If you have 990 MT, you have two internal 3.5” hdd bay for which Dell stock psu has two precut sata power connectors.  There are two external 5.25” bay for which there are another two precut sata power connectors.  You can convert those two 5.25” to 3.5” using adapter.  MT motherboard has total 4 sata ports, black, blue, two white.  The white are slow speed sata 2.0 reserved for odd which is also good for mechanical hdd (SATA 2.0 will not lower the speed of the HDD and SATA 3.0 will also not make the HDD faster).  The black blue are high speed sata 3.0 ports good for ssd.  So in summary 990MT max 4 sata drives (2 sata 3.0 + 2 sata 2.0).

DT and SFF have 2 sata 3.0 + 1 sata 2.0, USFF has 2 sata 3.0.

990 tech guide (p.14) https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/us/optiplex-990-tech-guidebook-final-1-4.pdf

You can install more than 4 sata drives if you use a PCIe sata add in card and a sata power splitter or upgrade psu to have >4 power connectors. 

Thanks for response.

I'll probably look at getting a sata add in card and power splitter. Didn't know they were an option, so i'll research it.

All info given is spot on so far.

In any HDD bay and the 5.25 DVD bays with hot swap bays or HDD bays installed, and if you use an adapter, you can fit two 2.5" drives (whether HDD or SSD) in each slot.  And you'd need the PCIe SATA port multiplier card redxps mentioned.

I think you'll find hot swap bays more convenient than just HDD bays that get covered up in the DVD bays.

I use:

• Orico Not Swap Rack 
• Sanrent External HDD Bay  There's models with and without fans.

You may find handy:  Dual SSD/HDD mounting bracket 

If you explore external options, there's many HDD/SSD docks, whether simge, dual, or multiple.  Be sure to use USB 3.0 slots.

Correction:  990 doesn't have USB 3.0 slots.  However, it can be added with a USB 3.0 PCIe card:  USB 3.0 PCIe cards 

And in the front:

Front & rear USB 3.0