I have just replicated this bug successfully on intrepid: $ uname -m i686 $ apt-cache policy zsync zsync: Installed: 0.5-1ubuntu3.8.10.1 Candidate: 0.5-1ubuntu3.8.10.1 Version table: *** 0.5-1ubuntu3.8.10.1 0 500 http://at.archive.ubuntu.com intrepid-proposed/universe Packages 100 /var/lib/dpkg/status 0.5-1ubuntu3 0 500 http://at.archive.ubuntu.com intrepid/universe Packages $ zsync -i karmic-desktop-amd64.iso http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso.zsync #################### 100.0% 771.4 kBps DONE reading seed file karmic-desktop-amd64.iso: *************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************Read karmic-desktop-amd64.iso. Target -26.2% complete. ******************************************************************************************************************** downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso: -------------------- -51.5% 594.6 kBps ETA downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso: -------------------- -77.8% 548.7 kBps -115:-57 ETA zsync works by writing an output file of final size at the beginning, and it does not seem to plan to get the whole DVD here: $ls -n total 2542652 -rwx------ 1 1001 1001 729829376 2009-10-15 10:30 karmic-desktop-amd64.iso -rw------- 1 1001 1001 2083430400 2009-10-16 08:50 karmic-dvd-amd64.iso.part it aborts: $zsync -i karmic-desktop-amd64.iso http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso.zsync #################### 100.0% 771.4 kBps DONE reading seed file karmic-desktop-amd64.iso: *************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************Read karmic-desktop-amd64.iso. Target -26.2% complete. ******************************************************************************************************************** downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso: -------------------- -51.5% 594.6 kBps ETA downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso: #################### 100.0% 683.3 kBps DONE 0 ETA Aborting, download available in karmic-dvd-amd64.iso.part verifying download... zsync is no longer running. Never telling the result of "verifying download..." let's see what we have now: $ls -n total 2812628 -rwx------ 1 1001 1001 729829376 2009-10-15 10:30 karmic-desktop-amd64.iso -rw------- 1 1001 1001 2147483647 2009-10-16 08:54 karmic-dvd-amd64.iso.part that is longer than the initial output file and the exact length described in bug 412413. Just to make sure the file should be longer: $ wget --spider http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso Spider mode enabled. Check if remote file exists. --2009-10-16 09:47:47-- http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso Resolving cdimage.ubuntu.com... 91.189.88.39 Connecting to cdimage.ubuntu.com|91.189.88.39|:80... connected. HTTP request sent, awaiting response... 200 OK Length: 4247142400 (4.0G) [application/x-iso9660-image] Remote file exists.