Here’s my experiences with upgrading from Ubuntu 11.04 to 12.04 on virtual machines (VirtualBox and Parallels), on a MacBook Pro.

VirtualBox

My initial setup was Ubuntu 11.04 running on VirtualBox 4.1.14 (the free/OSS VM software from Oracle/Sun).  Upgrading to 12.04 requires first upgrading to 11.10, so after snapshotting my VM, I attempted upgrading to 11.10 using Ubuntu’s GUI Update Manager.  Although the 11.10 packages all downloaded, and the upgrade nearly completed, the Update Manager went into a strange loop and kept popping up error windows, and the UI become unusable, forcing me to restart the VM.  On bootup, the VM froze at the Ubuntu splashscreen, so I restored my pre-upgrade snapshot and retried the upgrade process several times, each time failing identically.  After some investigating, it appeared that postgresql packages were causing the problem, so I used Synaptic package manager to find and uninstall all postgresql packages.  I then tried upgrading again to 11.10 — this time, the upgrade seemed to complete successfully, but on restart, the VM froze again at the Ubuntu splashscreen.  At this point I didn’t want to waste any more time, and decided to try out different VM software.

Parallels

The two alternatives to VirtualBox are of course VMWare Fusion and Parallels.  After reading some reviews, I decided to go with Parallels (I’d had a pretty good experience with an older version several years ago).  Although VirtualBox is open source (+1), it’s currently owned by Oracle (-1 (for their patent trolling)), so the thought of switching away didn’t bother me that much.  I was able to pretty easily import my VirtualBox VM into Parallels (using the ‘Add Existing Virtual Machine‘ feature, and selecting the appropriate .vbox file).  I then upgraded the Ubuntu install on the VM (running on Parallels) to 11.10 without incident.

I then upgraded to 12.04.  After the reboot to complete the 12.04 upgrade process, there were two problems:

1. The error message “an error occurred while mounting psf” (meaning broken Parallels Shared Folders)
2. The mouse not working in the VM

Fixing these problems required reinstalling Parallels Tools (without using the mouse!).  Luckily some keyboard shortcuts came in handy: after rebooting with the fresh 12.04 upgrade, use Ctrl-Alt-t to open a terminal window.  Then cd /media/Parallels\ Tools/, and run sudo ./install --install. Finally, sudo shutdown -r now, and you’ll reboot into a fully working Ubuntu 12.04 setup.