Today’s continuing testimony in the iTunes antitrust lawsuit has revealed that the company added changes to iTunes that deleted music that had been purchased through competing stores like Real Player from iPods. Users would not be notified that any music would be deleted by updating their music players.

The deletions allegedly occurred between 2007 and 2009, the Wall Street Journal reports. Whenever a user tried to sync an iPod with iTunes, it would check for music from other sources. If any was detected, iTunes would error out until users restored their iPods to factory settings. All music would be deleted during the process, and the iTunes application wouldn’t sync back music from outside libraries.

Apple claims that these measures were intended as a security measure to protect the user and device. Furthermore, Apple’s lawyers stress that while such security measures did exist, the plaintiffs have yet to produce a single case of music being lost. Security and user protection have been at the center of the company’s argument during this case, while the plaintiffs say that Apple acted in a monopolistic manner and violated antitrust laws.

Yesterday the jury saw a videotaped deposition of Apple co-founder Steve Jobs and saw emails between Apple executives regarding the decision to block Real Networks’ media from its music players. Phil Schiller and Eddy Cue are still expected to testify.

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