Today, as soon as I switched on my Xubuntu 16.04.4 computer, one CPU core went to 100% and there was constant HDD I/O for over 2 minutes. I looked at the system log afterwards and there were lots of lines such as:

os-prober: debug: running /usr/lib/os-probes/50mounted-tests on /dev/sda3
10qnx: debug: /dev/sda3 is not a QNX4 partition: exiting
20microsoft: debug: /dev/sda3 is not a MS partition: exiting
50mounted-tests: debug: running subtest /usr/lib/os-probes/mounted/83haiku
os-prober: debug: running /usr/lib/os-probes/50mounted-tests on /dev/sda4
50mounted-tests: debug: /dev/sda4 type not recognised; skipping

I found a reference to this on the Ubuntu forum saying that this happens after updating Grub, but I haven't updated Grub. The last time I updated anything on this system was about 2 weeks ago.

Please can someone explain what this is and why it happened?

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