Pentax K-7 Operating Manual page 278

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MSC and PTP
MSC (Mass Storage Class)
A general-purpose driver program that handles devices connected to
the computer via USB as a memory device. Also indicates a standard
for controlling USB devices with this driver.
By simply connecting a device that supports USB Mass Storage
Class, you can copy, read, and write files from a computer without
installing a dedicated driver.
PTP (Picture Transfer Protocol)
A protocol that allows transfer of digital images and control of digital
cameras via USB, standardized as ISO 15740.
You can transfer image data between devices that support PTP
without installing a device driver.
Unless otherwise specified, select MSC when connecting the Q
to your computer.
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