Data, as it happens in law, can also be owned. That's why we have a discipline called "Intellectual Property". It is a "thing" - like other properties like cars, houses etc., someone owns it.
Now, does the transfer of a computer also includes a transfer of data ownership stored inside the computer? Practically speaking, this clause is very unlikely to be included in the agreement when a computer is sold. It didn't explicitly say "yes" (you may use anything you find in the hard drive), but it didn't explicitly say "no" either, hence the question.
Looking at data, we can list a few legal concepts related to them:
- Data collection
- Data storage
- Data usage
The exact details vary by jurisdiction, but in general:
- Data must be collected through legal means
- Data must be stored in a secure manner to protect the data subjects
- Data must only be used by purposes agreed during data collection by the data subjects
- Copyright is not transferrable without signatures
- "License to use" is not transferrable either
Example 1, let's say a software is installed in the computer, with an appropriate license. This license was purchased by the original owner, therefore the software company licensed him to use the software, not you. The agreement does not have your name on it.
Example 2, the original owner is a photographer and have some beautiful photos stored on the hard drive. By virtue of Copyright, he is the author and copyright owner of these photos. There has been no signed document describing the transfer of copyright ownership to you, nor license you to use these photos in any way.
Example 3, the computer belongs to a company, and client information is stored on the hard drive. By transferring the computer to a third-party without wiping the drive clean, the company fails to use reasonable measures to protect the data of their clients. If somebody uses this data, then he is using data collected through illegal means, and can be prosecuted.
A counter example would be someone transferring ownership of the computer for the purpose of transferring the data inside. Note this is different: it is clear to both parties that the purpose of the exchange is to obtain the data stored inside the medium.
The ownership of the storage medium and the data inside are separated. Likewise any intellectual property associated to a physical object. When you go to the store and purchase a Bluray of a movie, you buy the physical ownership of the plastic disc + permission (license) to view the contents of the disc in a private setting. You do not buy the copyright of the movie even though it is in your hand.
To conclude, the answer is No - transfer of ownership of a storage medium has nothing whatsoever about the transfer of ownership of the data inside.