SanDisk Ultra USB 3.0 Flash Drive 64 GB - Unable to save certain JPG files

It sounds like the flash drive can save the pictures, "The size of the used memory has increased by 25 GB, " so the problem isn’t a storage problem.  Without knowing what OS you are running, what app you are using to view the pictures, what format you formated the drive to, or any error msgs you are getting it’s hard to say what the problem is but I suspect it’s your antivirus app causing the problem.