I would recommend checking the health of your sdb drive just to see if that is possibly the issue. You can do that from Ubuntu by doing the following:
First do the following to save the current health status parameters of your HDD to a file on your desktop:
sudo apt-get install smartmontools
sudo smartctl -a /dev/sdb > ~/Desktop/sdb_health_before_test.txt
That command will immediately terminate while the HDD begins its self-test, and it could take quite a while. You can monitor the progress with:
sudo smartctl -t long /dev/sdb
Once the above command says the test is done, then do:
sudo smartctl -a /dev/sdb | grep -A 1 -i "self-test execution status"
And please post the contents of the two files on your desktop so we can see the results.
sudo smartctl -a /dev/sdb > ~/Desktop/sdb_health_after_test.txt
sudo smartctl -H /dev/sdb