I would like to buy a Chromebook. Can I use memory cards to expand its internal 16GB of memory? Can I use a external hard drive that connects via USB port? What about an external DVD player, will that run on a Chromebook to watch movies?
Regarding internal upgrades (replacing SSD), it can be done, but the details of how to open the case etc will vary from model to model.
External USB storage is supported. The Chromebook doesn't really know or "care" what the storage technology is (Flash memory, HDD etc), as long as the drive presents itself as a standard USB Mass Storage device. However, the storage device must adhere to common standards: FAT32 or NTFS format; no "clever" software or special drivers required and the drive must not exceed 500mA power rating of USB 2.0.
As for playing DVDs, the answer is no. Playable DVDs are not merely a collection of video files -- they are organised in a very specific way and require a special player, and the Chromebook hasn't got such software.
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Supported file types and external devices
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Does chromebook support usb external HDD? (advice directly from Google experts)
Does chromebook support external dvd player?