file is damaged and can’t be opened. You should move it to the Trash.

Well the "sum" results for both are exactly the same, yet Catalina says the file is corrupt and Mojave is happy with it, which doesn't seem possible. So perhaps the attributes are what is causing the problem?

Wonder what would happen if you removed all the special attributes from the install_backblaze.dmg file -- perhaps you could try the following and then try and mount the dmg file:

xattr -d com.apple.lastuseddate#PS install_backblaze.dmg 
xattr -d com.apple.macl install_backblaze.dmg 
xattr -d com.apple.metadata:kMDItemDownloadedDate install.backblaze.dmg
xattr -d com.apple.metadata:kMDItemWhereFroms install_backblaze.dmg

Does that make any difference on the ability to mount the install_backblaze.dmg file?

Is it possible to give an explicit example of a file (dmg, app, etc) that fails for you, that I could try and download on my Catalina 10.15.2 system to see how my results compare to yours? If so, please give a URL and will give it a try and let you know how things turn out.

I just tried to download a copy of MacTracker.app from https://mactracker.ca and it worked fine, so the problem is not present for all Catalina 10.15.2 systems (the only file attribute on the Mactracker.app package was the com.apple.quarantine with a value of 57 (so none of others that you had on your install_backblaze.dmg file).

Good luck...

Also, I just created a new admin user "Test", logged into that user, and downloaded the Backblaze installer file from the link above. When attempting to open it, I get the same error message: “install_backblaze.dmg” is damaged and can’t be opened. You should move it to the Trash.

So this does not seem to be limited to my user account on this system. Bummer.

Any help you can provide to resolve this issue would be appreciated.

Well, I downloaded the URL onto my Catalina 10.15.2 and it mounts fine. The dmg file is the same size that you show but does not have the same attributes. These are the attributes I see on my Catalina 10.15.2 system (note that the first two attributes are added as soon as the dmg file is mounted):

user@My-Mac Downloads % ls -lae@ install_backblaze.dmg 
-rw-r--r--@ 1 user  staff  17658479 Jan  4 17:28 install_backblaze.dmg
	com.apple.diskimages.fsck	      20 
	com.apple.diskimages.recentcksum	      78 
    com.apple.lastuseddate#PS	      16 
	com.apple.metadata:kMDItemDownloadedDate	      53 
	com.apple.metadata:kMDItemWhereFroms	     101 
	com.apple.quarantine	      57 
user@My-Mac Downloads % sum install_backblaze.dmg 
3423 17245 install_backblaze.dmg
user@My-Mac Downloads % df -kt /Volumes/Backblaze\ Installer 
Filesystem  1024-blocks Used Available Capacity iused      ifree %iused  Mounted on
/dev/disk6s1  29300    25976      3324    89%     178 4294967101    0%   /Volumes/Backblaze Installer

The only thing rather out-of-the-ordinary on my system is a special SIP setup. It's not disabled but it has the following status:

user@My-Mac Downloads % csrutil status
System Integrity Protection status: unknown (Custom Configuration).

Configuration:
	Apple Internal: disabled
	Kext Signing: disabled
	Filesystem Protections: disabled
	Debugging Restrictions: disabled
	DTrace Restrictions: disabled
	NVRAM Protections: disabled
	BaseSystem Verification: disabled

This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.

You might try disabling SIP to see if that is the cause of this problem, but that is up to you, as that has other security implications. If you want to try, boot up in Recovery mode and issue the command in a Terminal window:

csrutil disable

and then reboot in normal mode. Note that this is a one time change, in the sense that another reboot will put SIP back in the default, enabled, mode.

Good luck...

Dot.com, you are a genius! As you suggested, I restarted my iMac in Recovery mode, launched Terminal, then invoked the csrutil disable command to disable SIP, quit out of Terminal, then restarted to my system volume running macOS Catalina 10.15.2. Once the system was restarted and I logged into my user account, the "file is damaged and can't be opened" behavior was gone. I was able to run the software installer for TechTool Pro (an installer package) and it worked perfectly. I was able to download and open PDF attachments from Apple Mail, all without that annoying message.

Thanks so much for your help.

I just want to emphasize: Don’t forget to enable SIP (System Integrity Protection) once all is complete. Use this command in Terminal while in Recovery Mode: csrutil enable

Then Confirm it is Enabled:

To confirm that SIP is enabled, reboot into Normal Mode and type this in Terminal (Found in Applications > Utilities): csrutil status

On my 2013 27" iMac, turning off SIP was the solution to my systems problems after macOS 10.15.2 was installed, at which point, everything in the Downloads folder gave the error message “file is damaged and can't be opened”.   This behavior was also true for any attachment downloaded from Apple Mail, any downloaded installer file (.dmg or package).  After SIP was disabled via the Terminal command csrutil disable in the Recovery mode, these symptoms disappeared completely. When I enabled SIP once again by rebooting my system, this anomalous behavior returned, with the "file is damaged" message being displayed (for files that were not, in fact, damaged at all).

Couldn’t find the post I was reading but, from another post I found, I believe I had to enter the following -

spctl --master-disable (Also I understand that the space after spctl is important so don’t omit it)

You *think* it was that? So...

What does the command do?

What are the risks?

How do you make it safe again?

This thread is becoming a tutorial on how to take down every safeguard they built into the OS. May as well run Windoze.

Hello TheLittles, after re-reading your post, I rebooted into Recovery mode in Catalina 10.15.2, enabled SIP using the csrutil enable command, then restarted in to Normal mode. Once there, I used Terminal to very that SIP had been enabled using the csrutil status command, then went about testing my system for the anomalous behavior listed in my other posts on this thread ( "file is damaged and can't be opened" ), and to my pleasant surprise, the anomalous behavior seems to be gone. I can download items from the internet (.dmg. .pdf. .jpeg) or from my backup drives and they open (or mount, in the case of the dmg files) just fine. I can open attachments from Apple Mail, and they open fine. Thank you (and all of the other contributors on this thread) to helping me resolve this vexing issue. Cheers!

I may have spoke too soon. It mostly seems to work. However, any file I now download from the internet has that problem. It says it is damaged. If xattr -cr on the file I can get it to work and open. So it seems the files are fine, but some kind of extended attribute is being added at download which breaks the file. Could SIP be corrupted? It seems like it is reading files wrong.

I encountered this issue this morning at a client's office when working on their two laptops. A MacBook Air and a MacBook Pro. Both were running macOS Catalina 10.15.3.

The MacBook Pro would not open any downloaded .DMG files saying they were damaged (as per this post). The MacBook Air was fine accessing the same files.

I discovered that all the downloaded files had been given a com.apple.quarantine flag in the xattr. After removing the quarantine flag all files were able to be opened normally. The command used was xattr -d com.apple.quarantine * from within the ~/Downloads directory

This however does not address why they were flagged in the first place, nor preventing new files from being flagged upon download.

I'd be very interested if someone else has some insight into this.

Following a full systems restore (after hardware repair) and thus OS upgrade to Catalina 10.15.3 I have this problem seemingly across all files. Disabling SIP fixes this, but it recurs when SIP is re-enabled.

Suggestions?

Hi Dazzzz, I can confirm the same happens on my system. I also get two HD icons in the finder. I have not found a resolution other than to keep SIP off. This happened even after a full reinstall fro scratch.

Thanks for responses. Certainly getting long, if not stale. Symptoms experienced are same at end of thread as those described by others at start. Problem is seen not just on my Mac, but also on others. Occurs across various file types and apps. Can be alleviated by changing OS settings (but a rightly pointed out not solved. First seen after an OS upgrade. So......