将 writeToFile: 原子地覆盖数据？

Yes. writeToFile:atomically: will.

Method writeToFile:atomically: ALWAYS OVERWRITE FILE, regardless of the atomically: YES or NO.

Information from Apple docs (NSData, section "Saving Data"):

The NSData class and its subclasses provide methods to quickly and easily save their contents to disk. To minimize the risk of data loss, these methods provide the option of saving the data atomically. Atomic writes guarantee that the data is either saved in its entirety, or it fails completely. The atomic write begins by writing the data to a temporary file. If this write succeeds, then the method moves the temporary file to its final location.

No one word about checking if file exist.

For example, for method copyItemAtPath:toPath:error: in docs (NSFileManager, section Discussion) specifically written about such checking:

If a file with the same name already exists at dstPath, this method stops the copy attempt and returns an appropriate error.