“The operation couldn’t be completed”/”Operation canceled” error message when saving to a Samba share via Files app

There are actually two messages you're referring to:

"This operation can't be completed because the item is in use"

and

"The operation can’t be completed because you don’t have permission to access some of the items."

If you see the first one - I sometimes get that message too. Quite often this can be resolved by running: sudo lsof | grep <filename> in Terminal.app - obviously replace <filename> with the actual name of the file you're trying to delete. I typically get this when trying to empty the trash, but if the trash is not supported on the filesystem you have mounted, you may see it when trying to delete.

What lsof does is it lists all open files, and shows which process opened each of them. Quite often this will be a quick look process that opened the file and somehow didn't close it (it will do that when you display a list of movie files in Finder - it will then try to show you the first frame of the movie in the preview, and it's using Quick Look to do that).

Usually after running the lsof the files are closed (that's if lsof doesn't show anything), or I can just kill the quick look process and then empty the trash.

As for the other message - the best thing to do is to run ls -la <filename> or ls -lad <directory> to see the permissions on the file or the folder you're trying to remove. This may give you a better idea where the problem is.

I've had similar problems compared to what you're describing, although it sounds like your problems are worse. It's very common based on what I've read and there's no cure. The issue is that Apple isn't using the Samba software but rather their own version. Here's my experience, FWIW:

1. SMB volumes were slower for me on Mavericks and older on these machines. Upgrading to 10.10.2 has sped it up and made everything work much better.

2. The shares are still very slow. One very large share performs much like you're describing at times. But other smaller ones are fairly managable, albeit frustrating.

3. We use Path Finder to connect to our shares, not Finder. This may be superstition on our part, as I don't know if it is actually doing anything different. But it does seem to give us fewer crashes.

4. DF in the terminal does not create the issue you have, it just lists like normal.

5. I have upgraded the RAM, but the hardware is older. One Mac Pro 2008 model has 8GB while another has 16GB. I've seen machines have a slow finder when they don't have enough RAM, but the problem is the software. Unfortunately, upgrading the RAM was the only solution I had to improve performance.

6. Windows shares may not list as slowly as my Linux NAS, but I have no benchmarks to prove that. (That would make sense if both weren't using Server Message Block.)

If none of this information helps you, may I ask if you've tried a fresh install?

As for me, I'm very interested in compiling Samba from source providing I can find enough support.