The link says connected, the speed says 100 Mbps. One bad pair in the cable, a forced duplex setting or green Ethernet is behind nearly every case.
Read **LinkSpeed** in **Get-NetAdapter**, then check the cable and the **Speed & Duplex** advanced property. Gigabit needs all four pairs, so one damaged wire drops the link to 100 Mbps.
**LinkSpeed** is the result of autonegotiation between the adapter and the switch, not a setting. 100 Mbps on a gigabit port means one side refused the faster mode.
Get-NetAdapter | Select-Object Name,Status,LinkSpeed,MediaType,FullDuplex
Get-NetAdapterAdvancedProperty -Name 'Ethernet' | Where-Object DisplayName -match 'Speed|Duplex|Green|Energy' | Select-Object DisplayName,DisplayValueA cable with a damaged pair produces errors even when the link reports 1 Gbps. Rising CRC or packet errors point at the physical layer, not at Windows.
Get-NetAdapterStatistics -Name 'Ethernet' | Select-Object ReceivedPacketErrors,OutboundPacketErrors,ReceivedDiscardedPackets
Get-NetAdapter -Name 'Ethernet' | Select-Object DriverVersion,DriverDate,InterfaceDescriptionGigabit needs all four pairs; 100BASE-TX needs two. A cable that works but only at 100 Mbps almost always has a broken pair or is Cat5 with damaged shielding.
:: swap in a Cat5e or Cat6 cable, then read the speed again
Get-NetAdapter -Name 'Ethernet' | Select-Object LinkSpeed
:: try another switch port as well, and a different switch if you have oneA manually forced 100 Mbps entry survives driver updates and is the classic cause of a link that never reaches gigabit. Set it back to autonegotiation and let both ends agree.
Get-NetAdapterAdvancedProperty -Name 'Ethernet' -DisplayName 'Speed & Duplex' | Select-Object DisplayValue,ValidDisplayValues
Set-NetAdapterAdvancedProperty -Name 'Ethernet' -DisplayName 'Speed & Duplex' -DisplayValue 'Auto Negotiation'
Restart-NetAdapter -Name 'Ethernet'Green Ethernet and energy-efficient Ethernet negotiate lower power by lowering the link rate on some adapters. Disabling them is the test, and leaving them off is the fix when they misbehave.
Set-NetAdapterAdvancedProperty -Name 'Ethernet' -DisplayName 'Energy Efficient Ethernet' -DisplayValue 'Disabled' -ErrorAction SilentlyContinue
Set-NetAdapterAdvancedProperty -Name 'Ethernet' -DisplayName 'Green Ethernet' -DisplayValue 'Disabled' -ErrorAction SilentlyContinue
Disable-NetAdapterPowerManagement -Name 'Ethernet' -ErrorAction SilentlyContinueThe negotiated speed is a claim; throughput is the measurement. Copy a large file from a gigabit peer and watch the counters, or use a throughput tool between two machines.
$a = (Get-NetAdapterStatistics -Name 'Ethernet').ReceivedBytes
Start-Sleep -Seconds 10
$b = (Get-NetAdapterStatistics -Name 'Ethernet').ReceivedBytes
'{0:N1} Mbit/s' -f ((($b - $a) * 8) / 10 / 1MB)Compare the driver version and the advanced properties of both adapters. The slow one usually has a forced duplex or a power-saving option enabled.
Thermal throttling in a USB or M.2 Ethernet adapter, or a switch port negotiating down under load. Check the adapter temperature and errors.
The bottleneck is elsewhere: the disk, the SMB share or the CPU. Measure the adapter counters during the copy before blaming the network.
USB and some virtualised adapters do not expose it. Those negotiate in the driver and cannot be forced from Windows.
Sometimes, on short runs and with all pairs intact. It is not rated for it — replace it rather than diagnose it.
Only when the whole path supports the same size. A mismatch produces stalls that look like a broken connection.
Set the advanced properties back with Set-NetAdapterAdvancedProperty using the previous DisplayValue, re-enable power management with Enable-NetAdapterPowerManagement, and restart the adapter.
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