I have a phantom 3 pro. I went to fly the other day and after i changed to the second battery the drone said the imu needed updating. When it got to 49% the update quit. I proceeded to update again and at 1% said unknown error. It will not update past 1%. This is the first time this has happened. The drone has never been crashed.

Hi bobbycokeman, I feel sorry about this has caused you. About the stucked to 1% are you referring to the IMU calibration process. If so, during the calibration ensure that the aircraft must be placed on level surface before the IMU calibration is conducted.
Have you check or refresh the firmware for your Aircraft. Update/refresh your Aircraft's firmware by following this video:

Meanwhile, here's the link for the firmware from the website:https://www.dji.com/phantom-3-pro/info#downloads just select the ZIP file. If the issue persist, send the Aircraft for test. You can refer to this link to create a repair case online: https://repair.dji.com/repair/indexThank you.