Two T10 Torx screws hold the hard drive in place.

Once they're removed, the hard drive easily pivots up and out of the system.

The 27" iMac ships with a 1TB 7200-rpm Serial ATA hard drive (Apple offers an optional 2TB drive for an additional $250).

There isn't really room in here for a second drive, but you could certainly swap this one out yourself. You could hypothetically install an SSD, but this seems like such a good a media computer that it would be a shame to limit it to current SSD capacities.

This + that = easy Time Machine internal backup!