Hard disk are really made up of magnetic materials but not like tapes....

A harddisk contain rotating very tiny plates known as platters....When we have to read or write an information from these plates (platters)they are bring close to read/write head which applies magnetic field on these plates hence the plates get magnetised and data is stored (thats why datas are not deleted as it is not easy to demagnetise the cells)
To read also the same procedure is used....
The whole system is rotated fastly so that we get the required flatter (small disk) as soon as possible thats why harddisks have RPM (eg: my harddisk speed is 7200.9 rpm )

Cd's are made up of a photosensetive material (which react towards light) protected by aluminum and fibre plates .......
A laser light (produced from GaAs laser diode is used ) is used to itch the photosensetive layer to write datas on it .....
An itched portion is known as "1" and unitched portion as "0"