--- title: "Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps" url: https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw source_url: http://feeds.hanselman.com/~/968151545/0/scotthanselman~Migrating-a-Synology-NAS-to-a-UniFi-UNAS-Pro-with-Robocopy-SMB-Multichannel-and-Surprising-Performance-Traps type: article source: "Scott Hanselman" published: 2026-08-23T21:54:08.092Z updated: 2026-08-23T21:54:32.046Z tags: ["windows", "networking"] reading_time: 15 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps **[Scott Hanselman](https://daily.dev/sources/hanselman)** · 15 min read · 0 upvotes · 0 comments ## Summary A personal account of migrating a Synology NAS to a UniFi UNAS Pro 8 using Robocopy over SMB, covering Alternate Data Stream copy failures, why /Z (restartable mode) tanked throughput, how /MT threading and /J unbuffered I/O interacted unpredictably with NAS-to-NAS transfers, and how SMB Multichannel let an old Synology's four 1GbE ports outperform expectations. The final recommended Robocopy command uses /E /MT:4 /R:2 /W:2 /COPY:DATX /DCOPY:DATX /XJ /NP /NFL /NDL, achieving about 187 MB/sec, versus roughly 67 MB/sec with rsync via UniFi Drive. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does robocopy fail with error 665 'file system limitation' when copying files to a UniFi UNAS Pro 8? The failure usually happens because the source file has an NTFS Alternate Data Stream (ADS), such as embedded album art stored as a separate stream like 01APIC_03.jpg, that the destination filesystem rejects. Running robocopy with /COPY:DATX and /DCOPY:DATX skips alternate data streams while still copying file data, attributes, and timestamps, resolving the error. _daily.dev surfaces practical fixes like this for developers debugging odd filesystem errors during NAS migrations._ ### Does the robocopy /Z restartable mode slow down large file transfers? Yes, /Z can significantly reduce copy performance because it adds extra logging overhead needed to make transfers resumable after interruption. In one Synology-to-UNAS Pro 8 migration over a stable 10 gigabit LAN, removing /Z alongside /COPY:DATX raised throughput to about 187 MB/sec for a 48 GB run with zero failures, so /Z is best reserved for unreliable connections rather than used by default on stable networks. _engineers tuning file transfer scripts can compare switch-by-switch performance notes like this on daily.dev._ ### Why would robocopy be slower when copying large files between two SMB network shares with the /J unbuffered I/O flag enabled? Unbuffered I/O via /J can dramatically slow NAS-to-NAS transfers when a Windows machine is simultaneously reading from one SMB server and writing to another, even though /J is generally recommended for large files on local disk-to-disk copies. Removing /J restored throughput to roughly 250 MB/sec in one Synology-to-UNAS Pro 8 test, showing buffered I/O worked better for that specific network path. _daily.dev helps developers dig into these buffering and I/O trade-offs before committing to a large-scale migration._ ## Similar posts on daily.dev - [My decade with Synology is over — a networking company just proved there's a better way to build a NAS](https://daily.dev/posts/my-decade-with-synology-is-over-a-networking-company-just-proved-there-s-a-better-way-to-build-a-n-qz71r1oq9) · XDA Developers · 0 upvotes · 0 comments - [I upgraded my NAS to 10 Gigabit and immediately discovered the bottleneck nobody warns you about](https://daily.dev/posts/i-upgraded-my-nas-to-10-gigabit-and-immediately-discovered-the-bottleneck-nobody-warns-you-about-wxz34mogx) · XDA Developers · 0 upvotes · 0 comments --- Tags: [#windows](https://daily.dev/tags/windows), [#networking](https://daily.dev/tags/networking) [View this post on daily.dev](https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw)