I continue to be plagued by this error. I've tried the following workarounds:

• Deleted the picture cache
• Consolidated the audio folder
• Changed DiscRecBuffers to 512 in Aud.ini
• Saved project to new folder
• Removed plugs

I've had occasional success but none of the above works consistently. The only thing that has worked every time is copying each track one-by-one to a new file but sooner or later I have the same problem. Does anyone have the definitive answer?
 
Thanks - rev 

Check you've not got anything else trying to access that folder, e.g: Windows Defender (or any other Virus Checker), 
Microsoft OneDrive, or any other Cloud drive.
 
So go into the config of all of these and make sure your Cakewalk Projects folder is excluded from any scan or sync.
 
[Edit] - it's not just your Cakewalk Projects folder that will need excluding, you'll also need to make sure your picture and Global Audio Folder are also excluded, and optionally any other folder in the "Folder Locations" listed in Cakewalk preferences.

If it's an HD (not SSD), make sure the offending drive is defragged.

I have no operations scheduled whatsoever, I don't use cloud drives, MBAM is my antivirus and it's the free version that does not have real-time protection. Is there a way of revealing what program might be accessing the folder?
 
I used Consolidate Audio Folder under Tools to defrag that folder but I did not defrag the whole disk. Does Sonar pre-allocate or limit space for a project via aud.ini?

revnice1
I used Consolidate Audio Folder under Tools to defrag that folder but I did not defrag the whole disk.

For project file cleanup, there's also the Clean Audio function under Utilities.
 
That said, neither of those have anything to do with Hard Drive Defragmentation.  If that hard drive has been in service for years and you've never defragged it, then it is long overdue. It will likely take many hours to complete, so best to leave it going overnight or when it is not being used for an extended period of time.

Hard Drive Defragmentation shouldn't stop Cakewalk from writing to the disk, unless it's so bad that it's timing out.
 
The usual suspects for getting this message (in order of likelihood) are:

• Some other process is accessing the file:  https://www.techsupportal...process-using-file.htm
• The disk has errors on it : https://www.thewindowsclu...ror-checking-windows-8
• The disk is actually full 

 
There's certainly no harm in defragging your disk, and I'd recommended you do, but IMHO it's the least likely to cause this issue.
 
 

I suggested a defrag because it's the only fix not yet mentioned here from this wiki list of fixes for the error.

Scan disk can sometimes help.

There have been number of threads on this over a long period of time, spanning many versions of SONAR. So far as I can recall, no one has ever really identified the root cause. It happened to me once with with one particular project. I found a thread at that time recommending to start playback momentarily before attempting to save, and that worked for me.

Not sure if its relevant, but one thought is to make sure your hard drive is formatted in NTSF, instead of Fat32.   Fat32 format has a max file size limit of 4gb (if I remember correctly).  Audio files can often grow to sizes larger than 4gb.  Formatting the hard drive in NTSF format allows you to have individual files larger than 4gb.