This is a common re-occuring problem with the GoPro Hero 9 and there are hundreds of complaints about it by now. However there is a work around. Here's some pointers: You can use either a 128GB or 256GB SD card, both card sizes are fine to use in the GoPro Hero 9. The problem has mainly been reported with Sandisk cards.
Latest GoPro models (including GoPro Hero 7) accept SDXC cards with a size limit up to 512MB. However, we always recommend to use GoPro compatible SD cards for best performance. 2. How to format GoPro Hero 7 SD Card? Make sure you backed up all your valuable data before formatting the SD card. With your GoPro on, swipe down on the camera VFJ2.