A:Answer Yes, the pre-installed 1TB HDD can be upgraded. The DVR can accommodate up to a 12TB internal SATA HDD 2.5" or 3.5" with a min. 7200RPM. How many days vary widely, based on where the cameras are installed, how many motion events they detect and many of the system's settings, such as resolution and number of connected cameras (this DVR can hold up to 12). Yes, you can view your cameras from a smartphone or tablet using the free Night Owl Protect App. Please view your product support page for more setup related information. Thank you! https://support.nightowlsp.com/hc/en-us/articles/360058760673