I opened GParted to look at the partition information of my drives. It popped up this strange warning:
The driver descriptor says the physical block size is 2048 bytes, but Linux says it is 512 bytes
I figured out that this error was about a USB thumb drive that was plugged into my computer. That USB drive was working fine and I could mount and read its files. So, what is this warning and how to fix it?
Turns out that the block size for this drive was set to 2048 bytes. It should have been 512 bytes. Its disk size was also appearing 4 times larger in GParted!
On thinking further about this, I remembered that this drive had been formatted as a bootable media to install Ubuntu using GNOME Disks as described here. That disk writer must have erroneously set the block size of this drive as 2048 bytes.
Changing back the block size to 512 was easy:
$ sudo dd if=/dev/zero of=/dev/sdc bs=2048 count=1
/dev/sdc with the device ID for your USB thumb drive.
Tried with: Ubuntu 16.04