Comment 3 for bug 743359

Also happens with 11.04 installer.

1. Created an unetbootin USB key from the release xubuntu-11.04-desktop-amd64.iso (md5 verified).

2. Booted the USB and selected Install Xubuntu in boot menu.

3. Chose to install the extra packages and download updates (wifi did not catch-up, so I plugged ethernet cable and installer detected net successfully).

4. Repartitioned manually all space as 256MB ext4 /boot + 4GB swap + 304GB btrfs /

5. Created user and chose autologon and no home encryption (using acer notebook disk lock in bios instead).

The installer crashed at "Checking for packages to remove".

After the crash, live XFCE was started, and after running installer again, it completed sucessfully.

According to syslog:
 plugininstall.py: File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 279, in _fetch_archives
 plugininstall.py: LockFailedException: Failed to lock /target/var/cache/apt/archives/lock

See /var/log/syslog attached to my original report at related (probably the same, but marked invalid due to possible hdd failure) bug:
https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/740558
https://launchpadlibrarian.net/72840690/syslog

Another related question: https://answers.launchpad.net/ubuntu/+question/152359