We have a virtual hosted Linux server, and in the last few months every now and then the HDD shrinks from 400GB down to the exact byte count that is in use. All existing data can be downloaded and displayed without a problem, but we can't upload or edit any files because of the "full" hard drive.

Here is a screenshot, where "size" should be 400GB:

This has happened twice before, and again today. The last times, when I reported the issue to the host, they said "that isn't possible, you must be doing it wrong", but soon after the call, the problem vanished without us doing anything, so I suppose that they have some kind of problem they're not willing to admit. Even after the fact, they acted like nothing was wrong and wrote me a mail in which they explained that I can use "df -h" to view available disk space (well duh, how do you think I noticed this particular issue?). Questions about if and what they had done were ignored.

It has happened around the 25th to 28th of the month, so I suspect that they might have a cronjob running every 30 days or so which wreaks havoc with some VM configs. I just want to understand the problem, but the host support hasn't been very helpful in that regard. I have tried Googling the issue, but any combination of search terms I can come up with just gives me tutorials on how to change HDD size in a virtual machine.

a) What could be the cause of shrinking HDD size in a Ubuntu 12.04.3 LTS server? Could there be anything in our virtual machine or is it more likely to be an issue with the vm host?

b) Can I do anything about it without needing to contact the host's support?

c) Is there anyway I can prevent this from happening at all?