GoKawiilUser troubleshoots Synology-to-UniFi UNAS Pro 8 NAS migration with Robocopy
A longtime Synology NAS owner attempted to transfer files to a new Ubiquiti UniFi UNAS Pro 8 over a 10-gigabit network, expecting the SMB file copy to be routine. Windows Explorer failed partway through with file system limitation errors, prompting a switch to Robocopy, which stalled at 92% with error 665 on certain files, including one called '05 Wires.m4p'. The user traced part of the slowdown to Robocopy's restartable /Z mode, a feature they had relied on since 2007 but which now dramatically hurt transfer speed.
GoKawiil's interpretation of the reporting above, not reported fact.
The account illustrates how seemingly simple NAS-to-NAS migrations can surface deep, decades-old assumptions about file copying tools that no longer hold true on modern hardware and network speeds. It's a useful reminder for IT admins and home lab users that legacy defaults, like Robocopy's restartable mode, can silently sabotage performance even when the underlying transport (SMB, 10GbE) is fast and modern.
- Windows Explorer struggled with file system limitation errors when copying between Synology and UniFi UNAS Pro 8 NAS devices.
- Robocopy repeatedly failed at 92% completion with Windows error 665 on specific files.
- The decades-old /Z restartable mode in Robocopy, once considered a best practice, turned out to severely slow down the modern migration.
Ubiquiti UniFi UNAS Pro — This is the exact rack-mount NAS the author migrated to, with multiple drive bays and 10G networking to match a fast internal LAN. If you've outgrown an aging Synology and already run UniFi gear, it slots right into the same console you use for your network.
See Ubiquiti UniFi UNAS Pro on Amazon → Affiliate link — we may earn a commission on purchases, at no extra cost to you. Product picked by AI based on this article; it is not a tested recommendation.Source: hanselman.com, 2026-08-24
Published there as: “Migrating a Synology NAS to a UniFi UNAS Pro 8 with Robocopy, SMB Multichannel”
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