Help-scrambled audio and noise when recording new files and playing back

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One of my external drives is suddenly acting up during playback of newly recorded audio. The files are playing back random audio from unrelated tracks, and white noise. It only happens when I record and then play back from this drive.

Recording and playback on my other drives works normally.

When I play these audio files by highlighting them and hitting the spacebar on the Mac, they play normally.

But if I import an audio file into a PT session on this drive, it also becomes corrupt during playback.

All of the previously recorded audio files (before today) play back normally.

Why would the playback of new audio files sound corrupt in a Pro Tools session, and not outside of PT? I thought it was a write problem on the drive but now I'm not so sure.

I've used this drive for several months without problems. It's a LaCie 4TB, run by a Mac 2007 Quad, running PT HD 9.0.6. It belongs to a faithful client and has a lot of sessions on it.

Any ideas? Thanks in advance.