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simon christo:
Hi there!! I have a 2 TeraByte Ext HDD, on my laptop, where the 4 Port HUB that is powered by a barrel connector (I have o check the voltage - but it's probally not 5V USB).
In case of Mains power failure, if it was pluged directly into the laptop, it would probally be the best senario, as the laptop battery would provide the nessesary delay, and "Shut Down Procedure" the HDD needs to shut down properly.
But using a Hub, brings in other questions.
I'd say that, without going through buying a Mains Powered UPS, just a simple DC-DC converter, would provide the nessesary power, to allow the Laptop to send the signal to the Hub to shut-down, if the laptop shuts down before the Hub power.

To cut a long story short, "How do you send a signal to the Hub that the power is about to go, and shut down properly??"

Thanks Simon.

Generally, unmounting/ejecting the drive is enough to flush any remaining data to disk prior to poweroff. The drive heads will self park after power loss. No signalling is necessary and there is no signal you send to the hub to notify it of a shutdown.

The hub probably uses a 5vdc power source at several amps depending on how many ports it has and what its advertised power to each port is. If you have USB C ports on the laptop they may be able to provide up to 3a. There are USB C boards you can buy that allow you to select the voltage/power it negotiates with the host and then tap that power into another device.

The OS already handles all of this.

It is the reason why you are supposed to never shut down a computer by just hard unplugging it. The shutdown procedure flushes out any data still left in the buffers, finalizes everything in the filesystem and also notifies devices of an impending shutdown. For USB devices a special message is sent out to inform them of the host system is going into sleep or shutdown.

The main use for these USB power management messages is to inform devices that they can go into a power saving mode since the 5V USB power tends to remain present even with the computer turned off (tho usually not on laptops). A lot of devices start pretending to be off when they see these messages until they get another message to inform them the host is up again.

This can also happen selectively. For example keeping a keyboard partially alive so that pressing a key can wake up the host from sleep.

Even without any of this, external USB disk drives tend to spin themselves down after a long period of inactivity.

First check what your system is set to regarding USB Caching.


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