There are two kinds of minidiscs. The prerecorded ones are purely optical, like CDs. The recordable ones are magneto-optical, so they’re read with a laser but written with both a laser and a magnet.
Even the recordable discs won’t be damaged by leaving them near a magnet at normal temperatures, IIRC. Each spot on the disc has to be heated up by the recording laser before its magnetic charge can be changed.
Correction: aluminum isn’t ferromagnetic, but it is weakly paramagntic. Paramagnetism is similar to the much stronger ferromagnetism we are all familiar with–the materials are attracted to magnets and can become weakly magnetized themselves. Parmagnetic materials, such as aluminum, platinum and uranium do have permanent dipole moments, but they are not able to completely align with an applied external magnetic field, the way ferromagnetic materials, like iron, do. Since it is thermal noise that prevents the moments from aligning, the colder a paramagnetic substance is, the stronger the effect.
Some experiments have shown, contrarily, that aluminum can also appear to be diamagnetic. Diamagnetic materials are weakly repelled but an applied external field. Bismuth is an example of a diamagnetic material. Surprisingly water is, too.
Aluminium is weakly paramagnetic, like oxgygen or copper sulphate (say). But it’s the electrical conductivity that’s important here, not the magnetic properties.
Simply put, pass magnetic flux lines over a conductor (by waving a permanent magnet over something metallic say) and an electrical current will be produced in the conductor. Wave a non-shielded speaker over a stamped aluminium layer CD, and a small current will be produced in the conductive metal. But not enough to do any damage.
Subject the same CD to a stupidly powerful moving magnetic field and it would be vaporised and propelled bodily at some great speed.
Really powerful magnetic fields will also kill you stone dead. They produce enough of an induced voltage in your body to disrupt everything at a cellular level. But you’re OK with speakers, shielded or not.
Correct. This principle is applied in may types of systems where oscillation damping is required, such as in mechanical balances. An aluminum plate on the end of the beam is made to cut through a magnetic field. This, as you say, induces an electric current in the plate. This current, in turn, creates a magnetic field which is opposite in polarity to the applied field, causing an attractive force which acts to damp the oscillation.