There are two problems with hubs, and they're the reasons hubs are so often dis-recommended (if that's a word). One is that in terms of throughput all of the USB connections on that hub share the bandwidth of the single port that the hub is plugged into. The other is that unless the hub has its own power supply, the maximum current that can be supplied is also split. These limitations can seriously impact performance, and in some cases even render a device unusable.
Hubs are not a problem with slow devices that don't need maximum responsiveness and have minimal power requirements: USB dongles, printers (if they have their own power) and computer keyboards. They may not be suitable for devices that have high current needs and/or lots of bandwidth: mice, DVDs, external disk drives. A MIDI controller, especially one that draws power from the USB port, would fall into the latter category.
Rather than look for a hub to use with your MIDI keyboard and USB drive, consider moving other devices to the hub instead to free up ports for them. If that's not possible, then at least get a powered hub (with a wall-wart power supply), and if your MIDI controller has the option of using its own power supply, use it.