When I wrote "This is a very worthwhile topic ..."
I didn't mean that it was a common problem, and everybody had a good chance of being bitten. (But, it is grossly misunderstood by the vast majority.)
... That is why MD5 and SHA-1 hashes are done on all evidence files.
I have processed hundreds of terabytes of data... some stored for a decade or more.... and never had a hash value change. I have never seen, or heard of from a reliable source, of any "bit rot" from a hard drive (optical media, yes). I've heard of corruption of a file at the file system level due to known hazards (and a diff of the two files shows it was corruption, and not bit rot), but never "bit rot" of a random bit flipping, and when you consider modern drives ECC, you realize that bit rot is not likely to manifest itself even if it happened.
You and I are in a very small minority. (I've been maintaining MD5s on all my media files [~12TB] for the last 5 years.) I haven't had any bit rot either. But, I have caught errors, following disk-to-disk, and network, copying of file(s). For the vast majority of users, without a means of verifying the integrity of the destination copy, those errors would typically go undetected; if/when
such an error does eventually get detected, it gets "blamed" on the disk drive it was read from, and incorrectly
categorized as "bit rot".
But ... every (current era) disk drive has bit rot! It is unavoidable, and it was planned for in the design of the drive and its firmware. In almost every occurrence, the firmware/ECC detects AND
corrects the "bit rot" and it never manifests in the real world. In those rare, but (wince!) dreaded cases where firmware/ECC detects but CAN NOT correct the error (serious rot), the drive issues the UCE (UnCorrectableError). Before issuing the UCE, the drive will make several (10+) attempts to get a correctable read. And, the kernel driver will make a couple of re-tries on the UCE it does get.
The super-elusive (and, maybe, mythologically apocryphal) case is where the "bit rot" is such, that, when ECC is applied to it, it (appears to) correct it, but actually produces data different from the original. [In NerdSpeak: a "Carl Sagan" "hash collision".] Probably as likely as three albinos, on separate continents, each winning their nation's lottery on the same day.
To [begin to] appreciate the complexities in modern disk drives (magnetic recording; not SSD), consider that there are several hundred data tracks packed (onto each surface) within the width of a single human hair!! (... my head hurts
...) That's about 200-300K tracks per surface on today's 3.5" drive. [Historical note: 40 years ago, there were about 400 tracks per surface on a 14" drive! [and only 10 sectors per track] (my first Unix driver