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I have a Western Digital 500GB passport plugged into my Raspberry Pi through a powered hub. The problem is that it seems to lose the connection and unmount every so often with the following dmesg:

[1208317.372126] EXT4-fs error (device sdb1): ext4_find_entry:935: inode #18096148: comm zsh: reading directory lblock 0**

It then automatically remounts as the hard drive is detected again (and it is configured in fstab). I then end up with ugly mounts like this:

/dev/sda1 on /my_passport type ext2 (rw,relatime,user_xattr,barrier=1)
/dev/sdb1 on /my_passport type ext2 (rw,relatime,user_xattr,barrier=1)
/dev/sdc1 on /my_passport type ext2 (rw,relatime,user_xattr,barrier=1)

...where /dev/sdc1 is in fact the only busy block device.

Thinking it came from the hub, I plugged it into my laptop through the powered hub. Unfortunately, the behavior is not reproducible even with intensive read/write access. I also tried unplugging the hub when it was connected to the laptop to make sure the USB power from the laptop didn't automatically take over. Basically, I am pretty certain that the problem isn't coming from the hub.

What's the problem here? How can I fix it?

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There are quite a few posts raising pretty much the same issues as yours I believe. Here are a couple. Most advice that you make 100% sure your power supply suffices RPI and the HDD needs. Not only your HDD needs to be supplied enough power but the RPI, too.

Disconnect everything from the RPI, leaving just the network and the HDD and see how this helps.

I don't know what your hub make/model is, but mine has two ports that provide 1.2A each whilst rest of the ports on it only do 500mA and will require a Y-cable to connect to HDD. Check into that to see if that helps.

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It was the USB hub causing the issue. I bought a new HUB and it fixed the issue.

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