If you have not tried it already start by force quitting the app and relaunching. If you still see the same issue uninstall and reinstall the app. If the issue persists after that power cycle the phone.
If all that still fails I would try formatting the drive. Connect it to a computer and if you have 32GB or less use FAT32 file system. if you have a 64 or higher use exFAT. if you are formatting on a mac make sure and use master boot record for the partition scheme.
If you still see the same issue I would suggest contacting sandisk support and submitting the debug logs so they can review. IN settings go to about and scroll to the bottom. Choose start capturing logs. Then try the transfer again. Let it set for a few mins. Then go back to settings about and choose send logs. You can send them to yourself. Once you have the logs open a support request with sandisk support and submit the logs.
