When recording footage from cameras to external storage, the default resolution is set at standard.
If you want to change to high resolution recording, go to the settings page for each camera, and use the "Enable HD" button.
If you find video for a particular camera corrupted or unstable after switching on "Enable HD", it means your WiFi signal to that camera is not strong enough to transmit at high definition and you may need to switch back to standard definition.
You can check how well your WiFi signal handles a high definition stream by live viewing the camera in question, and switching the live view to high definition using the LD/SD/HD button in the top left corner of the window. If the live view becomes distorted and broken at HD, then alert clips and recordings will be similar.
Please note, the "Enable HD" function controls both the resolution of continuous recordings, and of alert video clips.