Personal use copying was considered by Congress when it enacted the Audio Home Recording Act of 1992 (AHRA). The AHRA was a legislative compromise to deal with certain, specifically defined, categories of digital audio copying. Attempting to balance the various competing interests, among other things, the AHRA required the manufacturers of covered devices to (1) register with the Copyright Office; (2) pay a statutory royalty on each device and piece of media sold; and (3) implement what is known as a serial copyright management system (or SCMS) which prevents all but first generation copies. In exchange for this, the manufacturers of the devices, which might have otherwise found themselves subject to liability for contributory copyright infringement (among other things), received a statutory immunity from suit.
Consumers also received something. As long as the copying is done for noncommericial use, the AHRA gives consumers immunity from suit for all analog music copying, and for digital music copying with AHRA covered devices. It is important to note that the AHRA does not say that such copying is lawful; it simply provides an immunity from suit.
The difference between copying to cassette (for instance) as opposed to a computer hard drive is that audio cassette players (as well as Minidisc and DAT players) are devices covered by the AHRA and a computer is not. The specific reasons are technical but boil down to this: The AHRA covers devices that are designed or marketed for the primary purpose of making digital musical recordings. Multipurpose devices, such as general computer or a CD-R drive, are not covered by the AHRA. This means that they do not pay royalties or incorporate SCMS protections. It also means that neither the devices nor the consumers who use them receive immunity from suit for copyright infringement.