For months now, I’ve had a problem with this Snow Leopard-running iMac. I’ve been regularly updating it, done several repair permissions on the system disk, and nothing had fixed it. The problem is VERY annoying, and manifested itself this way: anytime I downloaded any disk image file (.dmg) and double-clicked it, it would not open, simply saying “No mountable filesystems”. Furthermore, my firewire hard drive would not mount automatically – I had to both “hdiutil attach -nomount /dev/disk1″ and “sudo mount -t hfs /dev/disk1s2 /Volumes/bitpool” every time I rebooted, and created a script to do this on reboot. At the time, I didn’t realize it was related, but camera memory cards would also not show up when I plugged them in to my reader. This reader had been having problems with it’s built-in iPhone dock, so I bought a new card reader – but the new one wouldn’t allow cards to be read either.

Well, I needed to import some pictures and was tired of plugging in the SLR and eating up its battery, so I did a “sudo mount -t msdos /dev/disk2s1 /tmp/card” … Aperture saw the pics, but didn’t show previews, and when I tried to do the import it failed, saying “/Volumes/card/EOS_DIGITAL…blah.CR2 failed to import” .. Hm! Aperture assumes whatever’s mounted must be under Volumes and have the volume name in it, huh? Ok, I’ll just create an EOS_DIGITAL directory in Volumes and manually mount it there… “Operation not permitted” .. Ok, SUDO mkdir .. “Operation not permitted” !! At this point, I started looking into ACLs, and ended up removing all the ACL entries on the /Volumes directory (figured it probably wouldn’t hurt, even if they were supposed to be there). Still no dice. I did another repair permissions, and it said the ACLs on /Volumes was wrong, but couldn’t fix it. Did some more searching and came across the chflags command, and “ls -lO” to see flags. My /Volumes directory had the uchg flag set! Again, didn’t know if that was supposed to be there, but killed it with a “chflags nouchg /Volumes”. Boom, mkdir works! I unplug my card reader and plug it back in… Boom, card shows up! I double click on a .DMG I hadn’t been able to open in months. Boom! I did another repair permissions, and it was now able to restore the ACLs on /Volumes – everything still works, so it must have been that uchg flag. I have no idea how it got there, but this difficulty was actually much harder to troubleshoot and solve than the issues I’ve had in Windows and Linux.

Here’s a very partial list of the google search terms I used to *try* to solve this, to no avail. Maybe this post will help someone else since I finally stumbled on the answer…
request from non-root process kextload “not allowed”
“failed to load” ufs.kext not privileged
hdid attach failed no mountable file systems dmg sudo
“mount_hfs” “permission denied”
mount_hfs hdid “attach failed”
(mount_hfs OR “mount -t hfs”) works (“mount -a” OR “mount -av”) ufs
(mount_hfs OR “mount -t hfs”) attach +nomount dmg
mac nothing automounts anymore
mac usb automounts anymore