This question seems like it would have a logical answer, but I thought I’d ask to be safe.
As shown in the photo below, there exist externally-powered USB hubs (A) and USB power-adapters—like the kind that come with some GPS and other USB devices—(B).
Connecting devices like keyboards, mice, flash-drives, MP3 players, etc. to the powered hub (A) is not a problem, as expected. The devices simply turn on (eg light up) or charge, or even do nothing as the case may be.
But, what happens when you plug a device into a USB power-adapter like (B)? I’ve plugged MP3 players (and of course the GPS device it came with) into it and they simply charged. But what would happen if I plugged a mouse or flash-drive into it?
Logic says that they would not be harmed and act exactly like they would if plugged into a powered-hub, but is that indeed the case? (unusual, proprietary adapters notwithstanding) USB connectors only have four pins, +5V, GND, +DATA, -DATA; so presumably only the power pins would be connected, and thus it would be harmless (as plugging it into any other live port would be). Again, this is excluding unusual, non-standard devices/adapters since they would not be compatible anyway (you can’t use the pins for whatever you want).
Am I right?
Update
Okay, so I took the chance and tried plugging a flash-drive into the adapter. My idea was to use the orange LED in the drive, plugged into the adapter as a cute little, low-power night-light. Unfortunately I over-estimated the luminosity of the LED (I could have sworn it was a lot brighter). The first shot is of the drive with some ambient light (a 12-ultra-bright-white-LED torch with 3xAAA), and the second is of the hallway with just the drive’s light (at night of course).
I tried a different drive and it didn’t even light up (because there’s no data communication). I then checked both drives with my computer and they were both fine.
Oh well, at least I learned something. :-)
Best Answer
Your logic is accurate; there is no inherent reason to expect harm. The device will just see a +5V source. Various power adapters have the data lines open, shorted, or connected through a resistor network to the power lines, depending on which of the standard or non-standard “this is a charger” schemes they use; but I believe the USB specification requires devices' data transceivers to be robust against this.
Possible hazards:
Some power adapters might be poorly regulated, or have higher voltage for faster charging of the device they are meant for. At a minimum, check the label on the power adapter to make sure it is specified to output 5V. (Higher current rating, i.e. current capacity, is not a problem unless it is a “linear” power supply, which is unlikely in any modern portable gadget.)
An especially poorly-designed peripheral might be damaged, not by the power adapter directly, but by being connected to a power source for a long time without being communicated with (staying in initial power-on state). This is unlikely.