I'm in the process of rearranging my home network and replacing a dedicated mini and an iMac with a 5K iMac to act as workstation and basic home fileserver. However, during the setup process, I've run into a really bizarre behavior that doesn't seem to make any sense. The configuration is thus: 5K iMac with Server installed has a folder on an external USB3 drive shared, with everyone given permission to write. Trying to connect to it using an older i7 iMac, and a fairly new Retina MBP. Everything is running Yosemite with all updates, network is wired Gigabit Ethernet with a new AC Airport and a Netgear dumb switch as the backbone. If I connect from the MBP, it behaves as expected--I can mount the share and write to it normally. If, however, I connect from the i7 iMac, every time I try to write a file to the mounted share, it forces me to admin authenticate in the Finder as if I didn't have write permissions. If I authenticate, it then complains that the file I'm copying already exists. If it is a single file (instead of a folder) and I tell it to keep both, I end up with two copies of the file--one with the correct name taking up 0K, and one with the name with a 2 appended that is the full file. If I disable SMB sharing for that share, it doesn't even show on the i7 iMac--it will only show if AFP is enabled. If I check the server, it's connecting only once, via AFP. Again, this is on a tiny network, everything is running the latest Yosemite, and it's all with the same (admin privileged) user. Rebooting both the server and the problem iMac made no difference. I'm taking the iMac out of service shortly, so I don't care that much about actually fixing the problem, but the behavior is so bizarre I do want to know what is going on. It is also quite similar to a problem I was seeing on one particular client machine at work, which I believe I eventually fixed with a complete reintstall of the OS. Any ideas here?