Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps

I’ve had a Synology NAS for a very long time, and recently I started moving its contents to a new Ubiquiti UniFi UNAS Pro 8. This seemed like it ought to be a fairly boring operation. Both devices speak SMB, I have a fast network (recently upgraded to 10 gigabit internally), and Windows has had tools for copying files reliably between machines for decades. Naturally, it turned into a whole evening of learning things I thought I already knew, which is why I started a blog lol. There was a nice bit of history here for me because back in 2007 (good lord!) I wrote a post called “XCopy considered harmful - Robocopy or XXCopy or SyncBack.” My argument at the time was basically that once you are moving enough files, Explorer stops being the move and Robocopy starts looking pretty good. I even used /Z, Robocopy’s restartable mode, because being able to resume a partially transferred file was useful on unreliable connections. Almost twenty years later, /Z turned out to be one of the most…