Full command string: RASPIVID -o test1vid.h264 -t 10000
File is saved. Updated Buster prior to installing camera. Reseated cable, swapped cable, swapped camera each with no change in results.
Command RASPISTILL -o image.jpg works correctly.
Using B4 With 4Gig.
Cameras are both Arducam Day-Night Vision for Raspberry Pi Camera, Wide Angle Automatic IR-Cut Switching All-Day Image, IR LED for Low Light and Night Vision, OV5647 5MP 1080P.