Do you develop npm modules on Mac OS X? Do you also use Time Machine to backup your work?

If you answered yes to both those questions, then there is a good chance you will be looking at this dialogue box at some point.

I ran into this problem a couple weeks ago. At first I thought my disk was going bad, but all the Disk Utility checks ran fine. Eventually I ended up looking in the log and found this gem:

Jan 15 22:25:37 xykon-2[38289]: Error: (-36) SrcErr:NO Copying /Users/bkelly/Dropbox/devel/node-netbios-session/node_modules/netbios-name/name.js to /Volumes/xykon backup/Backups.backupdb/xykon (2)/2013-01-15-222534.inProgress/9C56F536-65EE-46B1-8EF6-481D98533409/Corsair SSD/Users/bkelly/Dropbox/devel/node-netbios-session/node_modules/netbios-name

Its quite long, but if you look closely you can see that its complaining that it can’t copy a file from /usr/local/lib/node_modules to a node_modules directory in my development area. In particular, it was complaining about code that I recently had been referencing using npm link.

For those unaware, npm link is a handy command that lets you work with npm modules locally instead of installing them from the repository online. It does this by creating symlinks in /usr/local/lib/node_modules/ and the node_modules directory within a module you are working on. These symlinks point to another local module that may have edits that you have not yet published.

Of course, when you are done you may run npm unlink so that you can npm install the module from the repository again. This is where the problem can occur.

Unfortunately, it appears that Time Machine will sometimes get upset if you backup a symlink to a directory, convert the symlink back to a real directory, and then try to backup again.

Indeed, after I knew to search for symlink related problems, Google revealed that this problem is not new.

As mentioned in those other blogs, the solution to the problem is to delete the problematic directories, run another backup, and then replace the data. Fortunately, npm makes this fairly straightforward to do.

You can also add /usr/local/lib/node_modules to the Time Machine exclusions to try to proactively avoid problems.

tmutil addexclusion /usr/local/lib/node_modules

In fact, since the Time Machine exclusion is an attribute on the directory itself, its in theory possible to enhance npm or write a wrapper script to automatically set the exclusion whenever a new node_modules directory is created.

Of course, you may be saying to yourself that you do this all the time and have never had a problem. Well, just to make things fun, it appears that Time Machine only has problems with this situation periodically. In fact, I tried duplicating the error before writing this blog post and was unable to make it happen. Its unclear what other circumstances are required to trigger the condition.

So your workflow may work for some time and then seemingly break at random. Just be aware that it may be npm link related and try removing your node_modules before scrapping the backup disk.