To provide an on-vehicle hard disk device for reducing the time for updating data by using a write cache memory when updating data.

When writing the data stored in an external hard disk device 50 into a recording medium 32, a picture for prohibiting to power-off the on-vehicle hard disk device 30 is displayed on a display device 22, and the data inputted by a bus switch 38 is stored in the write cache memory 34. The data stored in the write cache memory 34 are written in the recording medium 34. After the writing of the data into the recording medium 34 is completed, a picture to permit power-off is displayed on the display device 22. Thus, because the picture preventing power off is displayed when writing the data through the write cache memory 34, the power is not turned off when writing the data.

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