Time ago, after noticing a progressive worsening in the overall performance of my iPhone 3G, I decided to do a full restore by reinstalling iOS 4.2.1 and then restoring from a previous backup, to see if the situation could improve a little. After deleting some apps I wasn’t using much and doing a backup, I was ready to go. I clicked on the Restore button in iTunes and after giving confirmation I was presented with a puzzling error message: The required disk cannot be found.

I never had a problem before with iPhone restores or firmware updates. I carefully read this Apple Knowledge Base page and tried every suggestion outlined there, to no avail. Then I searched the Web and lost myself in every kind of iPhone-related forum, only to find other puzzled users in my same situation. Then I found this page, promisingly titled Solved: The iPhone cannot be synced. The required disk cannot be found. It indeed gives good advice:

This sync error was happening on my iphone due to trying to sync the iPhone with a ringtone that was on a network drive that I recently reformatted and reorganized. Disabling the files that were on the network drive in itunes completely solved my problem. So if you ever read “The iPhone cannot be synced. The required disk cannot be found.” Make sure that all the files that you are trying to sync are actually accessible.

So I verified just that, that all I was trying to sync was truly accessible; I even deleted all the music, podcasts and videos on the iPhone to be sure. No joy. So, after losing an entire afternoon over it, and getting quite frustrated, I decided it wasn’t worth my time and just restored the iPhone 3G from a previous backup. That, and the ‘Reset all settings’ option managed to speed up the iPhone enough to be usable again, so I didn’t investigate further.

However, when yesterday I tried to update my brand new iPhone 4 from iOS 4.3 to 4.3.3, iTunes again threw the very same error at me. With a new, never-synced-before iPhone? Now, that was really strange. At first I thought it was a matter of activation, since I’m in the process of changing mobile carriers. I thought that perhaps I would need to wait to have access to Orange’s services (my new carrier). But a firmware update is a firmware update no matter what, so I started searching for the source of the problem. In this case at least I was sure it wasn’t something related to unreachable files because I hadn’t even synced the iPhone 4 yet. My instincts were telling me that iTunes was the culprit, so I started checking all files and folders related to iTunes.

It was when I navigated to Home/Library/iTunes that something dawned on me:

When I clicked on both the iPhone Software Updates and iPod Software Updates aliases, the Finder warned me that the aliases were broken. I keep my iTunes Library on an external disk, and a few months ago, when that disk was becoming unreliable and probably about to fail, I hurriedly moved the whole library on a newer external disk. So, when I checked the paths of those broken aliases, they were respectively:

/Volumes/[Old disk]/iPod-iPhone Software Updates/iPhone Software Updates
/Volumes/[Old disk]/iPod-iPhone Software Updates/iPod Software Updates

So I repaired them to be like this:

/Volumes/[Current disk]/iPod-iPhone Software Updates/iPhone Software Updates
/Volumes/[Current disk]/iPod-iPhone Software Updates/iPod Software Updates

Then I restarted iTunes and the error vanished. The iOS 4.3.3 firmware update was correctly downloaded and installed.

The bottom line: when you move your iTunes Library on an external disk, especially if you do it manually, make sure those aliases point to their respective folders on the external disk correctly, because iTunes saves the iPhone and iPod firmware updates there. If iTunes cannot find the correct path (in my case because it pointed to a disk that no longer existed), then it won’t proceed with the update process and will give you that fairly terse error message. (A warning like iTunes cannot find the ‘iPod-iPhone Software Updates’ folder would have been much clearer, in my opinion).