First of all, to my understanding, Tesla measures the flux density of a magnetic field (or the magnetic field itself). Interesting in this context is the magnetic force required to affect another magnetic field. This force is measured in Oersted.
It has been explained to me there is a relation between the two forces. As in, a magnetic field will also have a demagnetizing effect on another magnetic field. However, calculating the demagnetizing force is non-trivial (for me).
This fear is usually completely unfounded unless you're still using floppy disks for storage or are handling extremely strong magnets. The latter would have an effect on all kinds of electronic devices though.
There are numerous articles about this. The gist of it is, magnets won't do anything to your hard drives.
However, there are devices which can have an effect on data stored on hard drive platters, degaussers. These devices seem to be operate at around 5000 to 9000 Oe.
So, if you've gotten yourself a Q-51-51-25-N, you might have an actual chance to clear data off the hard drive. But you're more likely to squish your hand in between the drive and the magnet or cause pyhsical damage to the hard drive.
To give you some context, we see the degaussers mentioned above operate at over 5000 Oe. Another storage media that is often considered to be affected by magnets are magnetic stripe cards. Generally, these are available in 2 variants, HiCo and LoCo (high-coercivity and low-coercivity). LoCo stripes are usually erased by magnetic forces as low as 300 Oe. Even those aren't common in practice any more (exactly because of this weakness). HiCo cards are often available with around 4000 Oe.