Sometimes you need to put two internal hard drives in a system, but only have a single 3.5″ bay available. This is common in small computers, like HP Slimline and whatever Dell’s equivalent is.
There are two ways to do it. Both have tradeoffs.
It just so happens that many 2.5-inch adapters for 3.5-inch bays, like this one, have mounting holes for two drives. The downside with that approach is that you’ll have to replace your 3.5″ hard drive. And 2.5″ drives tend to be lower capacity and cost more than their 3.5″ equivalents. But they also use less power and make less noise. So if you need two separate physical drives, this trick lets you do it.
The other option, if you’re willing to jettison the optical drive, is to replace the optical drive with a hard drive in a 5.25-to-3.5″ adapter. You probably still use an optical drive sometimes, but you can plug in an external USB DVD burner for those occasions that you need to read or write CDs or DVDs. This approach allows you to keep your existing hard drive in place while adding additional storage. Since optical drives aren’t the kind of thing most people use on a daily basis anymore, this could be a viable option for some.
And, well, I lied. There’s actually a third option, if neither of these options is acceptable. If you can find room somewhere in the case, you can, in a pinch, place a 2.5″ hard drive anywhere there’s room and the cables will reach, and secure it with two-sided tape. It isn’t ideal, because the tape can fail due to the heat, and you need to take care not to stick the tape on the label of the drive–some drive manufacturers consider any foreign substance on the label as tampering, and void the warranty. Don’t block any ventilation holes and interfere with airflow. Make sure you have enough clearance between any electronics components, so you don’t create a short. And if any of this scares you, don’t use this third option.