How to add one more sata hard drive to my HP Slimline Desktop - 260-

There is 2 Sata ports on the motherboard of that system and one is for the current HDD and the other is your optical drive. It also will not fit another 3.5 inch drive. You would have to expand your storage externally with an external HDD or HDD enclosure via USB (look for a USB 3.0 drive as it will be the fastest).

HP Slimline Desktop - 455-012il PC Product Specifications | HP® Customer Support

Megahertz07 said:

USB flash disks has made ODD obsolete.
I would buy a 128 G SSD for Windows and programs and use the ODD SATA port.
If you just need more space, USB 3.0 external HDD is the best solution.

Then, other than using a slow, bulky, and clumsy external ODD, what would one use for listening to and ripping CDs and DVDs? What about those needing to load hardware drivers or an OS and their internet connection is too slow to make downloading impractical? CDs and DVDs usually have higher quality music and videos than downloads do. The "liner" notes on CDs are usually far more accurate and complete than the tagging on downloaded music. ODDs, while nowhere nearly as popular as they once were, are far from obsolete.

Lady Fitzgerald said:

Then, other than using a slow, bulky, and clumsy external ODD, what would one use for listening to and ripping CDs and DVDs? What about those needing to load hardware drivers or an OS and their internet connection is too slow to make downloading impractical? CDs and DVDs usually have higher quality music and videos than downloads do. The "liner" notes on CDs are usually far more accurate and complete than the tagging on downloaded music. ODDs, while nowhere nearly as popular as they once were, are far from obsolete.

+1 but unfortunately that is not the way things are moving forward.
@Megahertz07 That board may also only accept ODD's in that port as well since it's OEM.