Just for the record, I had all kinds of problems when upgrading CSS326. They are full of wonders.
First device was unable to upgrade. I was using manual upgrade procedure, because automatic one displayed "unable to download firmware". I downloaded correct firmware and tried to upload to css326. It said "do not interrupt", and it did that for about half a hour. I have tried many times with the same result. Finally, the device had rebooted itself and died. No lights, no link.
I have tried recovery procedure and it worked, the device had booted to firmware 2.0p. But still it was not possible to upgrade, and the switch was unable to boot normally (without recovery). It is dead for the moment, because I have no access to it.
For second device, I have tried to change my browser from Firefox to Chrome. Upgrade was successful this time. It takes literally one second to flash.
But then, when I had realized that 2.5 is too buggy, I had to downgrade. I made it a couple of times, until (when working from another PC) I had realized that the switch boots to recovery firmware 2.0p. Flashing via Chrome was unsuccesful then, every time it booted to 2.0p. But, this time, Firefox worked!