NAS transfer speed dropping to zero mid-copy is almost always caused by one of six things: a bad network cable, an SMB timeout misconfiguration, the NAS entering a sleep or power-saving mode during transfer, a failing drive, a network switch issue, or a resource bottleneck on the NAS itself. The good news is that each cause has a distinct diagnostic signature, and most are fixed in under 10 minutes. This guide works through each one in the order that accounts for the most cases.
In short: Start by testing with a different cable. If the problem persists, check whether the NAS is configured to spin down drives or enter HDD hibernation during transfer (turn it off temporarily for testing). If still dropping, run a network ping test during the drop to distinguish network loss from NAS-side issues. A sustained ping loss points to cabling or switch. A sustained ping with zero transfer points to the NAS OS or drive layer.
Before You Start: The Diagnostic Ping Test
The single most useful diagnostic step is to ping your NAS continuously while a transfer drops to zero. Open a command prompt (Windows: Win+R, type cmd) or Terminal (Mac) and run ping -t 192.168.x.x where 192.168.x.x is your NAS's IP address. Start a large file transfer, let it run, and watch the ping output when the transfer stalls.
If the ping also drops or shows high latency (over 100ms) when the transfer stalls, your problem is in the network layer - cabling, switch, or NAS network interface. If the ping stays at a consistent 1-3ms while the transfer reads zero, the network connection is fine and the problem is inside the NAS - drive sleep, OS bottleneck, or a drive error. This single test narrows the diagnosis before you spend time investigating the wrong layer.
Cause 1: Faulty or Poor-Quality Network Cable
A defective Ethernet cable is the most underdiagnosed cause of intermittent NAS transfer drops. Unlike a completely failed cable that prevents connection entirely, a marginal cable negotiates a connection and runs correctly for a few seconds before bit errors cause the connection to briefly reset. The transfer appears to stall. After a second or two, the connection recovers and the transfer resumes - or not, depending on how bad the cable is.
To diagnose: Swap the Ethernet cable between the NAS and your switch with a different cable you know is good. If the problem disappears with the new cable, the original cable was faulty. Even cables that look undamaged physically can have internal wire breaks from being bent or kinked. Try cables from a reputable brand - no-name patch cables sold in bulk can have inconsistent quality.
To check for errors: On Synology DSM, open a terminal session (via Control Panel, then Terminal and SNMP, then Enable SSH) and run ifconfig eth0. Look at the error and dropped packet counters - any non-zero values in these fields indicate the network interface is seeing errors. On QNAP, the same check is available through the Network and Virtual Switch manager.
Cause 2: Drive or HDD Hibernation
Both Synology DSM and QNAP QTS include a drive hibernation feature that spins down drives after a period of inactivity to save power and reduce wear. The drives take 5-15 seconds to spin back up when accessed. During a long transfer, if the NAS's CPU or network queue briefly stalls between file chunks, the drives can interpret that pause as inactivity and begin spinning down - causing the next chunk of data to wait for spin-up before the transfer resumes.
To check on Synology: Go to Control Panel, then Hardware and Power, then HDD/SSD. Look at the HDD Hibernation setting. If it is enabled and set to a short interval (such as 5 or 10 minutes), try disabling it temporarily and retesting your transfer. If the transfer completes without drops, hibernation is the cause.
To check on QNAP: Go to Control Panel, then System Settings, then Hardware. Look for the Drive Standby setting under the EUP (Energy Using Product) section. Disable or extend the standby interval for testing.
Drive hibernation is worth keeping enabled during normal idle use - it extends drive life and reduces power consumption. The fix is to extend the interval (30-60 minutes instead of 5), not to disable it permanently. A transfer that takes 20 minutes will not trigger a 30-minute hibernation window.
Cause 3: SMB Timeout and Protocol Version Mismatch
SMB (the network file-sharing protocol Windows uses to communicate with the NAS) has several versions and configuration options that affect transfer stability. SMB1 is an older version with known instability issues - Microsoft disabled it by default in recent Windows versions precisely because of security and stability problems. If your NAS is configured to use SMB1 only and your Windows machine has disabled SMB1, connections can appear to succeed but drop repeatedly during large transfers.
To check on Synology: Go to Control Panel, then File Services, then SMB. Ensure the minimum SMB protocol is set to SMB2 and the maximum to SMB3. SMB3 includes persistent handles that survive brief network interruptions and resume transfers. If you have SMB1 enabled, disable it. If the minimum protocol is SMB1, change it to SMB2.
Large transfer timeout: On Windows, large SMB transfers can time out if the NAS response is delayed. This is configurable in the Windows registry via HKLM\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters - look for SessTimeout and increase it from the default (if it exists). In most cases, upgrading to SMB3 and disabling SMB1 on the NAS resolves the issue without registry edits.
Cause 4: Network Switch Issue
An overloaded, failing, or misconfigured network switch can cause intermittent connection drops that manifest as transfer stalls. The ping test above distinguishes a switch problem from a NAS-side problem - if ping drops at the same time as the transfer, start investigating the switch.
Try connecting the NAS and your computer directly to the router (bypassing the switch) to confirm whether the switch is the cause. If transfers work reliably with a direct router connection, the switch is faulty, overloaded, or has a bad port. Try moving the NAS and computer to different ports on the switch. If the switch is unmanaged and several years old, a failing switch is worth replacing - quality 5-port or 8-port Gigabit switches cost under $50 and a 2.5GbE switch for NAS setups is available from brands including TP-Link and QNAP for $80-150.
Cause 5: Drive Health Issues
A drive that is beginning to fail will generate read errors during transfer. When the NAS's drive controller encounters a sector error, it retries the read - multiple times, each retry taking milliseconds. For a single bad sector, the delay is barely noticeable. For a drive with widespread surface degradation, those retries accumulate into a sustained stall while the controller attempts to read a large swath of problematic sectors.
To check on Synology: Open Storage Manager, then HDD/SSD. Check each drive's health status and run an S.M.A.R.T. quick test on any drive showing warnings. The Reallocated Sectors Count, Uncorrectable Sector Count, and Current Pending Sector Count SMART attributes are the most relevant - any non-zero value in these is a warning sign. A drive with pending sector errors should be replaced before it fails completely.
To check on QNAP: Open Storage and Snapshots, then Storage, then select a volume or RAID group and check the health status. QNAP's S.M.A.R.T. test is available per drive from the same interface.
If S.M.A.R.T. shows any Uncorrectable Sector Count above zero, treat this as an early drive failure warning. Back up all data to a separate device immediately and replace the drive before it fails completely. RAID protects you from a complete drive failure but not from a sector-level failure that corrupts data silently before the drive is removed from the array.
Cause 6: NAS CPU or RAM Overload
If the NAS is running other resource-intensive tasks alongside a large file transfer - a RAID rebuild, an antivirus scan, a Plex transcode, or a backup job - the NAS's CPU or RAM can saturate. When the transfer process competes with other processes for resources, it stalls intermittently while waiting for CPU time.
To check on Synology: Open Resource Monitor (available in the DSM application menu). Watch CPU usage during a transfer drop. If CPU is consistently above 80-90%, the NAS is overloaded. Identify which process is consuming CPU - a background antivirus scan or a RAID rebuild are common culprits. Schedule resource-intensive tasks for off-hours rather than running them simultaneously with large transfers.
On QNAP: Open Resource Monitor from the QTS desktop. The same CPU and RAM monitoring applies. QNAP's Resource Monitor also shows network throughput per interface, which helps confirm whether the bottleneck is network-side or processor-side.
Related reading: our NAS buyer's guide.
Use our free Transfer Speed Estimator to estimate how long large transfers will take over your connection.
Related reading: our NAS explainer.
My NAS shows full Gigabit speed for 30 seconds then drops to zero. What is the most likely cause?
This pattern - full speed then zero, then full speed again in cycles - most often points to drive hibernation or a marginal network cable. The cycling pattern suggests the connection is recovering after each drop rather than failing permanently. Start by swapping the cable, then disable HDD hibernation for testing. If the transfer runs cleanly after disabling hibernation, extend the hibernation interval to 30 minutes rather than disabling it permanently.
Does this happen more with large files or small files?
Drive hibernation causes drops specifically with long transfers (large files or many small files copied over several minutes). SMB timeout issues often manifest during transfers that stall briefly mid-file. Cable errors can occur at any file size but tend to be more noticeable with large files because the connection has more time to encounter a fault. If the problem only happens with large files, hibernation or SMB timeout is more likely. If it happens immediately with even small files, cabling or switch is more likely.
Will upgrading to 2.5GbE help?
Not if the problem is drive hibernation, SMB configuration, or drive errors - those affect transfers regardless of link speed. Upgrading to 2.5GbE increases maximum throughput from 125 MB/s to 312 MB/s, which matters when the NAS and drives are fast enough to saturate Gigabit but the link is the bottleneck. Diagnose the current problem first. A faulty $5 cable causing drops will not be fixed by a $100 2.5GbE upgrade.
The transfer only drops when other people are using the NAS simultaneously. Is this normal?
Yes and no. Some throughput reduction under concurrent load is normal - the NAS shares bandwidth and CPU across all connections. But a complete drop to zero is not normal, even under load. Check the NAS's CPU and RAM usage during the drop. If the NAS is maxing out its processor from concurrent requests, you may need a more capable NAS model or need to schedule heavy tasks (backups, Plex transcoding) during off-hours. On an underspecified NAS, multiple 4K streams plus a large copy can genuinely saturate the CPU.
I replaced the cable and disabled hibernation but the problem persists. What next?
Run the diagnostic ping test during a drop. If ping drops too, focus on the network layer - try a different switch port, a different switch, or a direct router connection. If ping stays solid but transfer drops, focus on the NAS layer - run a S.M.A.R.T. test on all drives, check for RAID degradation in Storage Manager, and look at Resource Monitor for CPU/RAM saturation during the drop. Also check the NAS's system log for any error messages around the time of the drops.
Can antivirus on my PC cause NAS transfer drops?
Yes. If your PC's antivirus is scanning every file as it is received from the NAS, it can throttle the transfer while it scans each incoming file. This is particularly noticeable with antivirus products that do real-time scanning with heuristic analysis. Try pausing your antivirus temporarily during a test transfer to confirm. If the transfer runs cleanly with antivirus paused, add the network drive or NAS IP to the antivirus exclusion list - files stored on a local NAS do not need the same real-time scanning as files downloaded from the internet.
Is your NAS not showing up on the network at all? That is a different problem with different causes. The NAS not detected guide covers the full diagnostic sequence.
NAS Not Detected Guide