External drives · Speed, cables and hardware
What does bus-powered mean, and why won't my drive work on some ports?
Short answer: A bus-powered drive gets its power through the cable. Ports — and especially unpowered hubs — can only supply so much, so some drives fail or disconnect on weak ports. Plug directly into the Mac or use a powered hub; desktop drives need their own adapter.
A bus-powered drive takes its power from the cable instead of a power adapter. Most portable SSDs and small portable hard disks work this way. Each port can supply only so much power, and hubs share theirs between everything plugged in, so a hungry drive can fail to start, disconnect, or click on a weak port.
What helps:
- Plug the drive directly into the Mac, not into an unpowered hub or a keyboard port.
- Use a hub or dock with its own power supply.
- Larger desktop hard disks always need their own power adapter — use it.
If a drive only misbehaves on certain ports, power is the likely cause. See whether to use a hub or plug in directly.