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When I try to open a DMG file I get this:

alt text

I'll just transcript the image:

There may be a problem with this disk image. Are you sure you want to open it?

Opening this disk image may make your computer less secure or cause other problems.

What does that mean in fact? What's really wrong with it, and what kind of problem can it cause just by mounting?

Someone said:

When you download a file in Leopard (and Snow Leopard), it's marked as a quarantined file. This occurs by the OS adding an attribute to the file, tagging where it came from (such as "downloaded by Safari"). This is what causes the user to see prompts when running files that were downloaded from the Internet, you may remember being asked to confirm you'd like to launch program XXX downloaded by Safari on XXX date. As a new part of Snow Leopard, files which are tagged with the quarantine attribute also have integrity checked by fsck, and if that verify fails you will see the message you described, triggered by an unused node in the disc image.

But really, I didn't get that. What's quarantine? I've just downloaded a file here on SL, tried to open, and got that warning.

Apple have a say about quarantine files, and they seem to work the same on Leopards. Plus I have got that file using Google Chrome while that feature seems to work just with Safari.

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Why was this voted down at least twice? I think it's a very valid question! –  Josh Mar 11 '10 at 14:50
    
You know @Josh, open community, anything can be voted up or down. Specially easier if votes are anonymous. I think it's nice. :) –  Cawas Mar 11 '10 at 16:32
    
@Cawas: I guess my point was, if there is something wrong with your question, it would be nice if the person who downvoted would comment as to what was wrong, giving you an opportunity to correct it. –  Josh Mar 11 '10 at 17:27
    
@Josh you should check one of others downvoted question. Specially look at that NSD guy: superuser.com/questions/109681/lazarus-form-recovery-for-safari –  Cawas Mar 11 '10 at 17:56
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@NSD I plan to one day learn how to answer this kind of rant on me in a funny way and without offending the offender. So, for now, I'm just sorry for anything that bothered you. Everything is always personal, but don't take it like that. –  Cawas Mar 12 '10 at 20:09
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2 Answers

up vote 4 down vote accepted

I am not sure about Snow Leopard, but in earlier versions of Mac OS X, this occurred if the filesystem on the volume failed an automatic fsck (filesystem check), and mounting the image might cause the Finder / Disk Image subsystem / other parts of the OS to crash, and/or cause more damage to the disk image causing further permanent data loss.

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I'll try to do a manual fsck and see if that's the issue. But there you go, this is good to know. It's really not ok to just ignore that message then. –  Cawas Mar 11 '10 at 17:44
    
I'll just quote the message fsck gave me here so I can research it later (hope this is not abusing the website :P) BAD SUPER BLOCK: MAGIC NUMBER WRONG ioctl (GCINFO): Inappropriate ioctl for device –  Cawas Mar 11 '10 at 23:31
    
@Cawas: I believe you should start a new question for that issue... Then way you have a question for how to fix bad magic numbers, because this question is about why you got the warning. If you'd prefer not to open new question I would recommend editing this one to reflect the new issue. –  Josh Mar 12 '10 at 0:21
    
@Josh I'll certainly do one of that, but do you think this question have been answered as it is yet? I think I'm still a little confused about all this. –  Cawas Mar 12 '10 at 20:33
    
@Cawas: What can I do to un-confuse you? :-) Edit your question and post what still confuses you –  Josh Mar 12 '10 at 20:50
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Josh is correct here. That is a generic error message when the mounting of the DMG may cause stability issues. It's not the result of an Antivirus or Malware check.

The quarantine flag that Safari places on the download is just a flag that is used to display the "This file was downloaded from the internet on date and.....". It's a reminder that the file was downloaded to the user, and to not necessarily trust the source... (Poor Man's Antivirus)...

The key here is that the original DMG was damaged, maybe not in any way that harmed the application stored on it, but the DMG was indeed bad...

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So this message and "This file was downloaded" message are basically the same? It doesn't seem very likely. And the flag wasn't set by my safari anyway, as I've used G chrome. As for a damaged DMG, well, if that's the case, I'd love that message to be more clear. It haven't even crossed my mind. I'll try to download it again, but I doubt that was corrupted in my download. Anyway, your answer did help me understand better, just by better wording that my quote. :) –  Cawas Mar 12 '10 at 13:09
    
@Cawas: He's saying there's two messages. The one you're seeing is because the DMG is damaged. The other, "This file was downloaded from the internet on.....", is a reminder that opening files downloaded from the internet may be unsafe. The two messages aren't the same. –  Josh Mar 12 '10 at 13:13
    
@Josh oh ok. Sorry Ben. I associated the "quarantine" he was talking about with the quoted explanation I brought, who said the problem is due to the quarantine flag. –  Cawas Mar 12 '10 at 13:42
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