I have a NAS drive attached to my router that is configured as Raid5. I also have three Mac's attached to that same router. In the Public folder on the NAS drive, I created a sparsebundle disk image from one of the Mac's. After it was created, it mounted to that computer just fine. When I try to mount it on another computer I get an error that says "no mountable file systems".

Basically, I need to have all three machines access the disk image (it has to show up as a local drive, which my NAS drive won't do) on all three machines. How do I get past this error?

One computer can use the DMG at a time by ejecting the disk image from computer 1 and then mounting it from computer 2 or 3. I really need all three to be able to use it at the same time. Final Cut Pro X (the software all three computers use most frequently) will not open libraries on a shared drive.

Thank you!

  • Can you unmount the dmg from the first mac and THEN mount it from another?
    – webmarc
    Sep 10, 2014 at 19:45
  • Yes. That works. But, it doesn't work for what we're doing. We all connect to it frequently and use files on it.
    – Terry
    Sep 10, 2014 at 19:51

1 Answer 1


I think the DMG probably needs to be served by an OS X AFP service that can do conflict management for when multiple computers try to change the filesystem at the same time.

Since you indicated in the comments that each of the computers can mount the DMG individually, but not simultaneously, I bet that your NAS is detecting that the DMG file has been opened for writing by computer1 and is locking it from open+write by other computers until the first is done (unmounts it).

Ooooh, according to this Q/A, you can convert a DMG to read-only; then use a shadow file to keep track of changes. Just make sure to have a separate shadow file per computer and you should be set!

Please test this out with a sample DMG before converting your 12 TB DMG. Steps:

  1. Create a trivial Read-Only DMG with a few files.
  2. copy it to your NAS
  3. In a terminal window hdiutil attach on /Volumes/NAS/path/to/your.dmg -shadow comp1.shadow
  4. head over to another computer
  5. run hdiutil attach on /Volumes/NAS/path/to/your.dmg -shadow comp2.shadow NOTE the different filename after -shadow, making sure that they don't both use the same shadow file.
  6. see if it mounted
  • Final Cut Pro X can't load libraries that are stored on a shared drive. So we need to trick the computers into recognizing the volume that's on the NAS drive as a local drive. Similar to an external plugged directly to a computer. We don't plug the NAS drive into one of the computers because it takes all the memory when that computer is accessed remotely.
    – Terry
    Sep 10, 2014 at 20:01
  • Gotcha, and the reason for not just copying it to all of the computers locally is that it's too large? Or something else?
    – webmarc
    Sep 10, 2014 at 20:12
  • Too large. The volume that we've created with the DMG is 12TB. The computers don't have that much space combined.
    – Terry
    Sep 10, 2014 at 20:14
  • Oof. My next suggestion was going to be replicating the DMG 2 more times on the NAS. If that's not feasible, I'm out of ideas.
    – webmarc
    Sep 10, 2014 at 20:18
  • Hey, can you mount it read-only from the first computer, or does it have to be read-write?
    – webmarc
    Sep 10, 2014 at 20:19

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