--- title: Migrating a Synology NAS to a UniFi UNAS Pro 8 with... description: 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... canonical: https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps | daily.dev og:description: 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... og:url: https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw og:image: https://api.daily.dev/og/posts/DRjU21MIW.png og:image:alt: Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps og:image:width: 1200 og:image:height: 630 og:locale: 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 - [Making Davinci Resolve Faster: Speeding Up SMB Reads From MacOS to Linux](https://daily.dev/posts/making-davinci-resolve-faster-speeding-up-smb-reads-from-macos-to-linux-s3o8qngpa) · scottstuff.net · 0 upvotes · 0 comments - [Debugging my new network, when 10 Gigabit Ethernet Runs at 300 Megabits](https://daily.dev/posts/debugging-my-new-network-when-10-gigabit-ethernet-runs-at-300-megabits-intohpmzt) · Scott Hanselman · 3 upvotes · 2 comments - [Automate File Syncs from Your Server to Synology with n8n and rsync](https://daily.dev/posts/automate-file-syncs-from-your-server-to-synology-with-n8n-and-rsync-tungpqfdf) · Noted · 3 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) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"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","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw"},"datePublished":"2026-08-23T21:54:08.092Z","dateModified":"2026-08-23T21:54:32.046Z","description":"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...","image":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","thumbnailUrl":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","isAccessibleForFree":true,"articleSection":"Scott Hanselman","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Scott Hanselman","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/e2763104a3c54967a617a207fd5f87ff","url":"https://daily.dev/sources/hanselman"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"windows,networking","timeRequired":"PT15M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Scott Hanselman","item":"https://daily.dev/sources/hanselman"},{"@type":"ListItem","position":3,"name":"Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel, and Surprising Performance Traps"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/migrating-a-synology-nas-to-a-unifi-unas-pro-8-with-robocopy-smb-multichannel-and-surprising-perfo-drju21miw#faq","mainEntity":[{"@type":"Question","name":"Why does robocopy fail with error 665 'file system limitation' when copying files to a UniFi UNAS Pro 8?","acceptedAnswer":{"@type":"Answer","text":"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. 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