I have noticed that two files on my (almost full) 2 TB hard drive have been corrupted. One file has 20 kB (!) corrupted, i.e. consecutive 20 kB have changed, even though the modification date of the file hasn't changed and I haven't worked with this file for over a year. This tells me that something "below" the file system level has messed with the data and the only thing I can think of is hardware failure, most likely hard disk failure.

I've tested my RAM already and it works flawlessly. I'm using ext4 on Linux, if that is of any help.

Is this normal?

Is it time to change my hard drive disk before something worse happens?

What can I do to prevent that from happening in the future?

Is there some built-in feature of, or extension to ext4 that includes additional error correcting code and/or watches files for changes that haven't been caused by the OS?