I've recently bought a Mac, and I want to share my Seagate FreeAgent 1.5 TB external hard drive with both my Mac and my PC. I've searched and found that exFAT or UDF is the way to go to have read/write access on both platforms. I've gone with UDF (I've read that exFAT is better for flash drives and it is also proprietary hence no official linux support -- if I ever need it just in case) and when I plug it in the Windows (where I formatted it as UDF) it works fine. When I plug it into my Mac, it says the drive can't be read and offers me to format it or eject it. When I go to disk utility, it shows the drive formatted as exFat (no I'm sure it is not. It is UDF, I formatted it from the command line with UDF option and it shows as "UDF" under Windows). I try to verify/repair the disk and here is the screenshot of what I get:


What could be the cause of the problem and how can I solve it?

  • 1
    Well firstly, trying to repair it won't work if it's wrongly detecting it. I'd wipe the drive, and format it under OS X as UDF, and see if that works.
    – user3463
    May 22, 2012 at 22:37

2 Answers 2


I'm working on this right now. The command line tools are there, but I can't figure out how to automount a UDF drive.

sudo mkdir /DriveName  # Don't put in /Volumes, so we don't interfere with the automounter
sudo mount /dev/disk2s1 /DriveName

Someone wrote a nice blog post about this: http://sipa.ulyssis.org/2010/02/filesystems-for-portable-disks/

The basic idea is to trick OS X into believing the whole disk is formatted as UDF while still having an MBR that's read by Windows.

There's also a Perl script (which works on OS X) for formatting drives that way.

  • schieferstapel, could you post the steps from the linked article into your answer? Links are good, but they may go bad. Posting the steps allows your answer to continue to help even after the link no longer works. Oct 31, 2014 at 21:53
  • The whole disk is already UDF formatted. Nov 1, 2014 at 19:42
  • @music2myear: I would, however there aren't any specific steps described there. There's a detailed description of how it works technically along with a script that can be used to partition/format drives under OS X. Here it is on an archive.org in case the page seizes to exist: web.archive.org/web/20140911062455/http://sipa.ulyssis.org/2010/… Nov 21, 2014 at 12:45

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