Really strange behavior creating a directory on SMB NAS share I am setting up a Synology DS918+ for backing up a new iMac Pro with an Drobo 5D3 attached to it.

I was working on backing up my Photos library from the Drobo using ChronoSync. ChronoSync was aborting with an error and after some debugging I can see what is happening, but its weird and I was hoping someone could help explain it.

To verify this is the issue I created a new SMB3 share on the Synology called Test and mounted it. Creating files and directories works as expected unless I create a directory named AUx.

$ ls -a
.       ..      .DS_Store
$ mkdir AUx
$ ls AUx
ls: AUx: No such file or directory
$ mkdir AUx
mkdir: AUx: File exists
$ ls -a
.       ..      .DS_Store   AHY9U3~9


Does anyone know what's going on?
I was having this issue, and I'm so glad that I'm not alone! I solved it by mounting the drive using AFP instead of SMB, and that meant that I could see files called AUX again. (For various reasons, renaming the directory was not an option.)