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Also, are you doing your downloads with the power adaptor plugged in or on battery? It's always safest to download software with the power adaptor plugged in, as you don't want your computer to run out of battery in the middle of a download.
You might try downloading your updates just a few at a time, or even one at a time for long ones or ones requiring restarts, and repairing permissions afterwards. It might also be prudent to close all open applications and do the downloads when you aren't doing anything else.
I am having the same problem with several computers (two iMacs and a MacBook), several Mac OS versions (Snow Leopard and Lion) on two differents Internet connections, is very fustrating I can't download iOS5 and XCode betas
¿Perhaps a problem with the Akamai network? I'm in Spain.
iMac 27" 2011 core i5
iMac 24" "white"
MacBook "white", core 2 duo
I have been seeing this intermitently with iOS updates and now with 10.7.3 updates - the download completes successfully but then the installer cannot complete because the file is "corrupt"
I strongly suspect there is something wrong with the file distribution folks - perhaps some setting akin to not setting binary mode on an FTP transfer - or not seeting passive mode etc.
Via software update on both a Mac Mini (client) and a Mac Mini Server - the update was about 700MB or so - and downloaded in under 3 minutes - but then failed to install.
The (client) cobo updater from Apple wbesite directly is 1.2GB takes abotu 20 minutes to download. will know in about a half hour if that works.