External USB hard drive momentarily disconnects when additional external hard drive is plugged in

Question

On a Windows 7 laptop with one USB 3.0 port and two USB 2.0 ports:

Whenever there is already an external hard drive plugged into the USB 3.0 port, and then another external hard drive is plugged into a USB 2.0 port, the drive that was already connected momentarily becomes disconnected, before automatically reconnecting again. Both drives are USB bus-powered hard disk drives; not SSDs.

The same is true if a drive in already plugged into a USB 2.0 port, and then another drive is plugged into the USB 3.0 port.

The above behaviors become apparent by assigning Device Connect and Device Disconnect sounds in Windows. The behaviors are confirmed by using USB Device Tree Viewer.

Both external drives are the same model, Western Digital WDBBEP0010BBK-01. These drives are compatible with USB 3.0 and USB 2.0.

According to the Windows 7 Device Manager, the driver being used is "WD My Passport 0748 USB Device" provided by Microsoft (Driver Date: June 21, 2006, version 6.1.7601.19133).

Is this normal? If not, how can it be prevented? Is data loss possible?

Answer 1

This can be normal behavior. I have seen it happen often enough on systems that I know are in perfect working order. It is no reason for alarm. But it is very irritating.

What happens is the following:
The USB system supplies up to 100 mA of power to each connected device. If a device needs more than that (and USB powered harddrives usually do) the device can negotiate a higher power-budget (up to 500 mA) with the USB controller.
The controller may or may not allow the higher power-draw depending on the total power it can deliver and what is already used by all other connected devices.

When you connect an additional device 2 things can happen:
- The device doesn't ask at all and just starts using power. Which isn't as intended but there is a lot of badly designed devices out there that just do that anyway. Most USB controllers start to supply more power automatically but this may result in a short drop of the voltage on the USB bus which could be the cause of the other device disconnecting and reconnecting.
- If the device asks nicely if it is allowed to use more power the controller can decide to ask all other devices if they are willing to reduce their power-consumption. Some devices can't really do a re-negotiation like that. They are only capable of negotiating on initial connect and they may respond to such a re-negotiate request by disconnecting and connecting again (simulating the initial connect).

Regardless of the exact cause there really isn't anything you can do about it.
Using a USB hub with its own power-supply between the computer and the devices could help as those hubs usually have a beefier power-supply than the USB controller in the computer.

Answer 2

Is this normal? If not, how can it be prevented? Is data loss possible?

It is not normal from technical standpoint, but it is quite common. The reason is that these drives are mechanical drives, and usually have high current consumption when spinning-up on initial plug-in. If you open your drive enclosure, you will likely see something like this,

As one can see, the drive itself uses 750 mA, which already exceeds nameplate capability of a USB 2.0 port. It is unclear what does it that exactly mean, sustained average consumption or absolute max peak, but experience shows that peak currents can be much higher, up to 1.5 A and above.

Notebooks in general don't have much luxury to supply plenty of power to external USB ports, especially if more than one heavy load is plugged in. As result the port power usually has a short dip when an extra drive gets plugged in, which disrupts USB interface on the running link and causes the abrupt port disconnect. And yes, "surprise disconnect" during some critical data transfer to HDD might cause drive corruption and severe data loss.

How to avoid these hot plug issues with bus-powered HDD enclosures? Use a self-powered USB-certified hub (good luck in finding one), with a solid external AC-DC adapter. Or use bus-powered SSD drives instead of classic mechanical spinners.

And don't listen to urban legends about classic power negotiations - standard USB system has no means to limit supply to 100 mA. See this post for more details.