I have a laptop set up with Kubuntu 12.04, 64-bit, and I used it for a couple months without problems. Then, after booting it up one evening and copying a batch of files onto the hard drive from a USB stick, the system froze. No soft-reboot would work, so I used the power button. After that, the computer does not boot to the GUI desktop, but only to console, and allows only root to log in.

Note that the files that were copied immediately before the failure were copied to a data partition separate from the system files. All those files are in place.

From the console, as root, I can run startx and get the GUI desktop. I cannot, however, use the su command to change to a regular user. I have tried changing to the user that existed before the failure, and I have created a new user since then and tried it. Neither works.

Obviously, the new user's password was created after the crash. And I've also used the passwd command to reset the pre-existing user's password, so I know it's valid.

The error message I get in both cases is (where the ellipsis represents the user's name). The /home directory has full drwxrwxrwx permissions, as do both users' directories. Both users' directories are owned by the respective users, and all files (including hidden files) have rwx permissions in the owner's column.

Any ideas where I can look further for solutions?

Can you check dmesg or any other log?

Moved: This thread is more suitable in <Ubuntu> and has been moved accordingly to help your thread/question get the exposure it deserves.

Yes, I can check /var/log/dmesg, though I have no idea what the result means. I don't even know if newer material is added at the head or the tail. Nor do I know what other logs to look at. Can you offer any guidance here?

we will need more information on this.
i suspect something went very, very wrong there.

this sounds wrong.
my /home has drwxr-xr-x root:root, and /home/username has drwx------ username:users.

...and the root account is disabled on Ubuntu.

ondoho, I don't know how much more information there is to give. I had several video files on a USB stick, mounted the USB stick on the laptop at /mnt/usb1, copied the files to /video (which is on a partition separate from the root and system files), and let it run. When I went back to it, an hour or so later, the system was non-responsive. Ctrl-C had no effect. Ctrl-Alt-Del had no effect. I held the power button down to shut the machine off, then re-booted.

The system, since then, boots to the console and asks for a login and password. If I enter a regular username and the associated password, the system will very briefly display a message - it looks like a "Last login was..." message, but it disappears too quickly for me to read in detail. Then it again asks for login and password. I can enter username root and root's password and the computer accepts those credentials and runs as root, in console.

When I try to su to another user, I get the error message that the system cannot cd to that user's home directory. However, I can manually cd to those home directories.

The permissions have not changed since I set the machine up. They are set the same now as when I was able to use the machine normally, before the freeze-up.

What filesystem is the main filesystem? If it was ext2 I could believe that it would corrupt something on hard shutdown.

All partitions on the HDD are ext3 file systems.

Was /video mounted?
Maybe your / is full?

Yes, it was mounted. The root-directory partition has over 25 GB of unused space. No partition, in fact, is anywhere near filled.

OK, well back to looking like user error to me ...

the actual commands you used.

but to me, too, it looks like you accidentally hosed your system.

and if it really was set up like this from the beginning, then something went wrong there, too.

After I plugged in the USB stick and determined the drive and partition to which it was mapped (sdb1), I did this:
I had previously made sure that the USB stick had nothing on it before I copied the video files (in their respective subdirectories) onto it, and I had checked that the /dev/sda5 partition was mounted to /video.

The files copied correctly, but the computer would not respond to anything after they were in place - except, of course, the power button.

I'm not sure what you mean by "...set up like this..." but the system was not initially configured to boot to console nor to allow a root log-in. For the first few months, it booted to a grub2 menu screen, defaulting to the Kubuntu OS and allowing my regular user-name log-in (but not allowing a root log-in, which is the norm for Ubuntu). It changed to the console-only, root-log-in-allowed behavior only after this freeze-up.

that looks ok to me.
of course one could discuss whether there was a need to perform that command while still in superuser mode; had you exited first, there'd simply have been less risk of accidentally doing something fatal.i meant the permissions on your home directory/ies, as you quoted earlier.

unfortunately, at this point there's not much anyone can do to help you unless you start seriously troubleshooting your problem (probably from booting into a live system).