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I have a 17" MacBook Pro (Late 2011 model). Whenever I copy a large file between them (> 1GB), the transfer stalls or fails outright after the 1st gig or so. It doesn't matter if the file is on the Mac or the PC.

When I attempt to copy a file from the Mac:
Activity Monitor's network tab shows bytes are flowing at the expected speed. But somewhere around the gigabyte mark (the exact spot is inconsistent), the transfer rate gradually slows and slows until it basically comes to a complete stall. The graph shows the connection with only small blips of 3 or 4KB every few seconds and will remain this way indefinitely. Finder's copy dialog just gives up trying to estimate the time remaining.

Activity Monitor no activity

Attempting to copy the file from Windows: Exact same behavior. Explorer dialog starts out showing the file copy at the expected speed and Task Manager confirms. The only difference is that Explorer will throw the following error when the dropout or whatever occurs:

Network Error in Windows

Here are the symptoms:

  • The dropout happens around the 1 gig mark. A 500 MB file always works. A 1.5GB file never works. A 1 GB file sometimes works and sometimes doesn't.
  • It only happens for individual files that are large. I can copy a large folder full of small files just fine.
  • Neither computer shows any other signs of network problems. I can browse shares on both machines during and after as though nothing had happened.
  • The issue only occurs if the Mac is on a wireless connection. If I hardline with an Ethernet cable, it works fine regardless of whether the PC is wireless or not.
  • It happens anywhere I go. I've tried it both at home and at work. At home I'm using an Apple Time Capsule as my router. At work everything is Cisco.
  • The issue occurs regardless of the direction of the copy (i.e. PC to Mac or Mac to PC doesn't matter).
  • Mac to Mac or PC to PC transfers on the same network work fine. I only have one Mac, but I can copy large files back and forth to my friend's Mac with no problem. I can also push/pull the file to/from my Time Capsule from my Mac just fine, and Windows has no problems there either.

What I've Tried:

  • I've actually had this problem since I've owned the computer and I just finally got annoyed enough to post the question. It's been through several reloads and a hard drive upgrade since then. But the story has always been the same through Mountain Lion, Mavericks, and now Yosemite.
  • I've tried several different PCs both at home and at work, with various versions of Windows from Win7, Win8/8.1, Server 2008/2012, etc. The behavior is the same.
  • Manually setting my MTU size to various different values, including 1472.
  • Disabled Antivirus on both sides.
  • Googling the problem yields no useful solutions. It's like nobody has ever had this problem before.

So has anybody fixed this issue before?

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  • Have you tried protocols other than SMB? What about enabling Remote Login on the Mac (ssh), install WinSCP on the Windows box and scp from the Mac to Windows?
    – MMB
    May 4, 2015 at 19:09

2 Answers 2

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According to the following Apple Support Community post, the fix might be raising the Session Timeout:

  1. On the Windows machine, open Regedit, and go to \HKLM\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters\

  2. Add a DWORD value called SessTimeout and set it for something like 3000 seconds

  3. Restart workstation service

https://discussions.apple.com/thread/4781793

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  • I could try that, but if it works then I'd have to change that on every PC I connect to, which isn't really a solution.
    – Wes Sayeed
    Apr 30, 2015 at 23:16
  • @WesSayeed - You can also try it, if it works, we determine a solution that will work but your unwillingness to even attempt it makes me not even willing to investigate further.
    – Ramhound
    May 1, 2015 at 0:20
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This issue appears to have been fixed in OS X 10.11 El Capitan. It is no longer necessary to raise the timeout value on the target Windows machine to facilitate large file transfers.

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