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My hard drive needs more space.

This sentence expresses the need for more storage space on a hard disk. It may be used when the hard disk is filled or nearly filled with files.

I'm going to buy a new hard drive.

This sentence is someone expressing their intention to purchase a new hard drive. This might occur when the current hard drive is damaged or insufficient to user's needs.

Your computer's hard drive is very fast.

This sentence comments on the speed of the hard drive in someone's computer. It might be used if files are being transferred or accessed at a high rate of speed.