Unfortunately, we're aware of an issue affecting users on macOS Ventura when trying to save to an external drive such as a flash drive that has a FAT partition.
You will be able to initially save to the flash drive as expected but any subsequent saves to the same file will give the error:
"Failed to save document. Save failed because access to the file was lost. The document must now be closed."
This issue is logged with our developers for further investigation but saving locally and to a non FAT partitions is unaffected.