iTunes syncing is broken: Apple, please fix it
Now, syncing an iOS device--iPhone, iPad, or iPod--is too often an ordeal. And it is because it's become untrustworthy. Will the sync work at all or will your content disappear and be transformed into something that fills the amorphous "Other" category in iTunes' capacity bar. Will all of your content sync or just your music, or music, or apps
Sync problems between iTunes and iOS devices are all too common. (See the last thirty days of posts in Apple's support forums about iTunes sync issues.) In a way, this may be a predictable side effect of Apple's push to online services. The company wants everything to be in the cloud, and it would prefer that you buy all your music and movies from there as well. Local syncing isn't really a part of that plan and so may be treated as an afterthought. The difficulty is that not all users are right for the cloud model. For those with large iTunes libraries, or with limited broadband bandwidth, cloud storage simply isn't usable.
Given that, it's time to revisit local syncing. In its current state, iTunes syncing is broken and it can only be fixed by Apple. Here are some of the most common sync problems, and, in some cases, solutions or workarounds to get media files onto your iOS device.
Waiting for items to copy
When you sync an iOS device, iTunes displays a number of steps as the sync procedure advances. One common problem is that iTunes gets stuck on the "Waiting for items to copy" or "Waiting for changes to be made" step. And this can take hours. (Here's one thread on Apple's support forums discussing this problem. Users offer several solutions, but most of them aren't definitive as they work for some people, but not others.)
iTunes is parsimonious in its communication with users--there's no log you can view, even in the Console utility that gives you access to system logs. If there were, you might be able to troubleshoot whatever's stuck. In some cases, a single, bad file can cause problems. If you know which file that is, you can remove it and potentially get the sync to work. (See below: It's the dupes' fault)
When the "Waiting for items to copy" problem happens regularly, there are two solutions you can try. The first is to wait, even overnight, if necessary. It might eventually sort itself out; but it might not. The second, more heavy-handed solution, is to restore the device, which is something I've had to do many times since the release of iOS 8.
To do this, start by backing up the device. Click on your device in the iTunes navigation bar and then click Summary. Near the top of the window you'll see a Restore [device] button, as below. Click Back Up Now, just below the Restore button.
Next, click Restore iPhone (or iPad, or iPod), and follow the instructions. iTunes will reinstall the device's software and then prompt you to restore from the backup. This can take a long time if you have one of those new-fangled 128 GB iPhones and want to put lots of music and videos on it.
It's the dupes' fault
Macworld contributor Rob Griffiths ran into an annoying sync problem with his new iPhone 6. He discovered, with the help of Apple support technicians, that duplicate purchased items in an iTunes library can cause syncing to stop. iTunes never alerts you to any problems, it just silently stops syncing. In Rob's case, there were about 20 such duplicates, and after he weeded them from his iTunes library, the sync worked; for the most part. Read Rob's account for more about this problem.
Album art won't sync
It's increasingly common to find that album art doesn't sync to iOS devices, or that certain songs or albums display the incorrect artwork. For the latter problem, the only solution I've found is to stop syncing music. To do that connect the iOS device, select its icon in iTunes, choose Music in the sidebar, uncheck Sync Music to the right, and click the Sync button. Then return and enable the Sync Music option and then click Sync again.
But there's one issue where album art won't sync because of its size. It seems that artwork larger than 10MB doesn't sync with music files. Admittedly, this is a very large image file, but it would be nice if iTunes could alert users to this limit.
Free space isn't, and Everything becomes Other
This problem seems to have appeared following the release of iOS 8 or iTunes 12. Many users are finding that iTunes shows a certain amount of free space in its capacity bar. Yet when they try to sync something, such as a movie or a lot of music, iTunes tells them they don't have enough space.
One iTunes Guy message I received recently summed up the situation:
Since the purchase of my iPhone 6 Plus, I've had to restore and re-sync my phone at least five times due to a mysterious issue that pops up whenever I attempt to add new music or videos to the device. When plugging in the phone to sync the additional content, the iTunes "Other" part of the capacity meter balloons from about 2GB to around 40GB. My 128GB iPhone 6 Plus tells me that my phone is over capacity. iTunes either refuses to sync, indicating that the iPhone is full, or it hangs.
This happened to me as well, in the brief time I had an iPhone 6, and happens, from time to time, with my other iOS devices:
A related problem, which has been around for years, is the amount of space reported as "Other" in the iTunes capacity bar. Here's a Google search for this problem on Apple's support forums, limited to the last 90 days, showing how many people are affected.
In some cases, all the music on a device somehow becomes "Other."
I suspect that these related problems are caused by some kind of database on iOS devices that gets corrupted. The device thinks it has a certain amount of free space and iTunes disagrees; hence the confusion. The Internet proposes a lot of supposed solutions for this one--that you should delete old text messages and browser caches, for example. But none of these solve the problem. The only solution I've found is to restore the iOS device, as explained above. But that doesn't appear to be a permanent fix. After a few syncs you may find that the Other space starts ballooning again.
What Apple needs to do
Apple needs to fix syncing. While users who don't sync their iOS devices in this way aren't affected by these issues, those people with small and large iTunes libraries alike report syncing problems. It's frustrating, and the fact that there's no way to find out what's wrong makes it even more so. In an ideal world iTunes would have some kind of sync log or sync diagnostic tool, akin to the Network Diagnostics utility, that would help ferret out problems and let people get on with enjoying their media.