In which directory are Android updates downloaded? I want to download and install a system update for my Android HTC Desire.

The download was successful but during installation it stopped. In the ClockworkMod console I can see the error "signature verification failed".

I wanted now to try to toggle signature verification in the ClockworkMod menu off and give it another try but unfortunately I cannot find the update file (ClockworkMod menu proposes /sdcard/update.zip but this file cannot be found).

Do you know where the updates are downloaded or any other hint to help me update my system?

(I used ClockworkMod v2.5.0.7 for rooting my system)
You cannot install an over-the-air (OTA) update from the manufacturer or carrier if you have ClockworkMod Recovery installed, even if you disable the signature verification.  You will need to return to the stock recovery console and stock ROM (if you are currently running a custom ROM) first, and then go to Menu -&gt; Settings -&gt; About -&gt; System Updates and re-start the update process.  Keep in mind that you will lose root once the OTA is installed, and you'll have to re-root your phone all over again.

Alternatively, you can see if there is a custom ROM available for your phone that is based on the same Android version as the OTA, such as CyanogenMod.  I'd start at XDA-Developers forums for your phone.

In either case, it's very likely that you will need to wipe your device's internal memory also (which erases all of your installed apps and settings,) so don't forget to backup what you want to keep.