Recovering Corrupted Downloads the Right Way

You don’t have an OC-48 or even T3 in your home. It takes you 24 hours+ to download the latest DVD image of Ubuntu or Windows Vista. Or maybe it “only” takes you 12 hours+. Either way, you’ve just finished your download to realize that its corrupt: the crc32 and md5 hashes just don’t add up. You burn it any way, only to find that it crashes randomly at some point of the install – or if you’re really unlucky, once it boots and seems to be working fine.

It’ll take you an entire extra day (and night) to re-download that copy.. and this time you’ll want to make doubly-sure you don’t disconnect in the middle or hit the power-switch by accident. Then you stumble upon this article, and realize you’ve been going about this the wrong way. Because the answer isn’t to download it again, but to only download the parts that are corrupt, the parts that you need – and nothing more.

The solution is bittorrent. Chances are, whatever DVD image or file you’re downloading exists on someone else’s PC. Given the huge number of people using bittorrent, it’s highly likely that if you’re downloading something popular, some kindred soul out there has chosen to share their bandwidth with the world to help out people like you.

Maybe you chose to download this file via HTTP (normal internet) because it was faster than waiting for that torrent with only 2 seeds and 2 peers to finish downloading at 12 KB/s. Maybe you didn’t know that it was available on bittorrent, or maybe you didn’t even know what bittorrent was. But it doesn’t matter, because now bittorrent can help you save that data without downloading it all again.

First, you’ll need a decent bittorrent client. µTorrent is great for just about everyone using Windows, Linux, or Mac (the latter two by using Wine). Just download it and run – no setup needed. Then you’ll need to search for the file you’re looking for. In this case, we’ll be searching for Windows Longhorn Server Beta 3, English Enterprise Edition. We already dowloaded the ISO from Microsoft just to find out it was corrupted somewhere in the past 14 hours of downloading. So we head over to IsoHunt and search for the file: wsl_6001.16510.070417-1740_x86fre_server-KB3SFRE_EN_DVD.iso. We’re in luck, and someone’s decided to share. We open the link in µTorrent, and check the file hash really quick against the one we got from Microsoft – and they match!

Now we’re all set. Just start the download in µTorrent, tell it to save it in the same location as the corrupt download, and it’ll do the rest. First, it’ll check to see if a file with the same name already exists in the same directory. Since it does, it’ll break this file up into parts (the size of each part varies according to the .torrent file in question), and check how many of these parts are identical to the original (complete & uncorrupt) ISO and delete those parts that aren’t. Then it’ll set about downloading the missing pieces (the ones that were corrupt, but are now gone) from the various peers online, and in a matter of minutes, you’ll have a complete and fully-functional file, ready for burning or mounting!

Since we’re nice folks, we’re going to leave µTorrent on and hidden in the task bar where it won’t use much bandwidth and close to zero resources and help out the next person that comes along looking for a piece or two — it’s only fair, isn’t it?

Bittorrent has in-built hash-checking which serves multiple purposes: first it authenticates every piece that comes in – just in case someone is sending you corrupt data or there’s interference along the way. Then it connects all the hundreds or thousands of little pieces together, and checks to make sure the final result is the same as the original. You’re protected on multiple layers, don’t have to download the entire file all over again, and can relax knowing bittorrent’s got your back!