The increased size of firmware updates is a necessary result of increased functionality but, when multiple iDevices need updating, the download times and bandwidth cap hits can be noticeable.
Thankfully, iTunes is smart enough to store the firmware update after it’s been downloaded the first time, and it will reuse that file to update subsequent devices of the same type on the same machine.
If you have multiple devices syncing to multiple computers, however, there’s a workaround that can help you avoid re-downloading hundreds of megabytes each time. Simply copy the firmware file from the first computer you used to update your device to any other computer that syncs with devices that need updating. The trick, however, is finding where Apple hides those firmware files.
On OS X, after a firmware update has been downloaded via iTunes, the firmware file can be found, depending on the device, in:
~/Library/iTunes/iPhone Software Updates
or
~/Library/iTunes/iPad Software Updates
or
~/Library/iTunes/iPod Software Updates
Starting with OS X Lion, Apple hid access to the user’s Library folder so you’ll need to either hold down the Option key while selecting “Go” from the Finder’s menu bar, or press Command-Shift-G to open the “Go to Folder” window and type the applicable folder location above.
On Windows XP, the iTunes firmware files, with the same iPhone/iPad/iPod folder name distinction as mentioned above, are stored in:
Documents and Settings\[User]\Application Data\Apple Computer\iTunes\
On Windows Vista and Windows 7 the location is:
Users\[User]\AppData\Roaming\Apple Computer\iTunes\
In order to see the Application Data or AppData folders, Windows users may need to enable “Show Hidden Files and Folders” in Windows Explorer > Organize > Folder and Search Options > View.
Locate the applicable file (with an .ipsw extension) and copy it to another computer (in the case of these firmware files, they’re OS-independent, so a firmware file can be copied from a Mac to a PC and vice versa).
Note, however, that each model of device requires its own firmware, so if you have an iPhone 4 and your significant other has a 4S, you won’t be able to use the same firmware file for both. In fact, there are even distinctions based on network type, with a GSM iPhone 4 not able to share firmware with a CDMA iPhone 4. Thankfully, if you make a mistake and attempt to update the firmware with an incorrect file, iTunes will stop the process and inform you that the firmware file is not compatible.
Hold Alt/Option while clicking “Check for Update” or “Restore” to select a pre-saved firmware file (Source: Apple HT4623)
Once the firmware file is on the second computer, connect the device that needs updating and launch iTunes. Select your device from the iTunes sidebar and, while pressing the Alt/Option key, select “Check for Update” or “Restore” depending on your preference.
Now, instead of iTunes reaching out to the Apple servers and downloading another copy of the device firmware, the update or restore process will proceed immediately, saving you time and bandwidth.