stop finder replacing colons with slashes in filenames

Solution 1:

It is historic. In Classic Mac OS, : was the path separator, not /. (Classic Mac OS itself copied the : as the path separator from SOS for the Apple III.)

OS X is based on Unix (technically XNU, with POSIX compliance), and so the Unix parts, such as the command-line tools, used / as the directory separator. However, the GUI and the actual file-system used on disks (HFS+) had to deal with the Classic Mac OS apps, which were using : as the directory separator, just as they always had.

Thus, for backwards-compatibility, the GUI layer (i.e., the Finder) does not allow : characters to be used in file names and Unix does not allow /. The Finder is responsible for mapping one to the other.

For more information, see ldav1s's answer on Stack Overflow, which cites a USENIX 2000 paper, "The Challenges of Integrating the Unix and MAC OS Environments" that describes some of this history.