I have a folder on an external drive connected via USB (K:\ drive). I was connected to my Raspberry Pi via WinSCP, and had started a Synchronize operation:
Target: Remote
Mode: Synchronize Files
Synchronize Options: Preview Changes
Comparison Criteria: Modification time.
Transfer type: Binary; Only update new and changed files.
The Raspberry Pi drive has 3.5T available of the 4.1T capacity.
The K:\ drive has 6T free of the 7.25T capacity.
System has 32GB of RAM and nothing is open except a web browser (Brave) and Win SCP.

All files are either .mp4 or .mkv files that I have backed up on my K:\ drive and am trying to restore to my Raspberry Pi drive.


WinSCP 5.21.5

Error message:
Copying files to remote side failed.

Invalid access to memory.

Stack trace:
(0004F645) ntdll.dll
(00FDC86E) C5412_0
(00FDE8D6) ____ExceptionHandler
(00FDB690) __ExceptionHandler
(00097BBF) ntdll.dll
(00077B51) ntdll.dll.KiUserExceptionDispatcher
(0008F358) ntdll.dll
(0004E2C1) ntdll.dll.RtlFreeHeap
(00025554) USER32.dll
(00023FC6) USER32.dll
(00025BB6) USER32.dll.EnumThreadWindows
(002393B4) Vcl::Forms::_16496
(000B41B7) TTerminalManager::UpdateAppTitle
(000B5290) TTerminalManager::OperationProgress
(00CC6F9A) TFileOperationProgressType::AddTransferred
(00DCF7E7) TSFTPUploadQueue::SendPacket
(00DC5306) TSFTPFileSystem::Source
(00DF6F2F) TTerminal::Source
(00DF5898) TTerminal::SourceRobust
(00DF5DE5) TTerminal::DirectorySource
(00DF6CA9) TTerminal::Source
(00DF5898) TTerminal::SourceRobust
(00DF56C5) TTerminal::DoCopyToRemote
(00DC1E7A) TSFTPFileSystem::CopyToRemote
(00DF5124) TTerminal::CopyToRemote
(00DF1302) TTerminal::SynchronizeApply
(0001D85E) TCustomScpExplorerForm::FullSynchronize
(0001E7AB) TCustomScpExplorerForm::DoFullSynchronize
(00E8E4D9) TSynchronizeChecklistDialog::OkButtonClick
(0027DE9B) Vcl::Controls::TControl::Click
(00282489) Vcl::Controls::TWinControl::WndProc
(001EA618) Vcl::Stdctrls::TButtonControl::WndProc
(002825EF) Vcl::Controls::_17076
(00282489) Vcl::Controls::TWinControl::WndProc
(00281AD0) Vcl::Controls::TWinControl::MainWndProc
(003C9C0C) System::Classes::_18201
(00031511) USER32.dll
(000222E1) USER32.dll
(00021D65) USER32.dll
(0002BC8D) USER32.dll
(00077B0B) ntdll.dll.KiUserCallbackDispatcher
(00037C13) USER32.dll
(00020DC1) USER32.dll.SendMessageW
(0007345A) COMCTL32.DLL
(00083BC2) COMCTL32.DLL
(00031511) USER32.dll
(000222E1) USER32.dll
(00021C52) USER32.dll.CallWindowProcW
(0028259A) Vcl::Controls::TWinControl::DefaultHandler
(00282489) Vcl::Controls::TWinControl::WndProc
(001EA618) Vcl::Stdctrls::TButtonControl::WndProc
(003C9C0C) System::Classes::_18201
(00031511) USER32.dll
(000222E1) USER32.dll
(00020BA0) USER32.dll
(000206EB) USER32.dll.DispatchMessageW

(00FDCED9) C5412_0
(00FDE4B3) _ReThrowException
(00DDC2F7) TTerminal::DoProgress
(00CC6F9A) TFileOperationProgressType::AddTransferred
(00DCF7E7) TSFTPUploadQueue::SendPacket
(00DC5306) TSFTPFileSystem::Source
(00DF6F2F) TTerminal::Source
(00DF5898) TTerminal::SourceRobust
(00DF5DE5) TTerminal::DirectorySource
(00DF6CA9) TTerminal::Source
(00DF5898) TTerminal::SourceRobust
(00DF56C5) TTerminal::DoCopyToRemote
(00DC1E7A) TSFTPFileSystem::CopyToRemote
(00DF5124) TTerminal::CopyToRemote
(00DF1302) TTerminal::SynchronizeApply
(0001D85E) TCustomScpExplorerForm::FullSynchronize
(0001E7AB) TCustomScpExplorerForm::DoFullSynchronize
(00E8E4D9) TSynchronizeChecklistDialog::OkButtonClick
(0027DE9B) Vcl::Controls::TControl::Click
(00282489) Vcl::Controls::TWinControl::WndProc
(001EA618) Vcl::Stdctrls::TB

The problem happens rarely. I cannot reproduce it.