I hate you! My external disk drive has 214.03 GB available. Now what?
I think what I’ll try is move all my existing backups from ~/Library/Application Support/MobileSync/Backup/ to my external drive and link the new location to the old location.
iTunes keeps telling me that there’s not enough space. 
Hours later… I’ve deleted ~/Library/Caches/* and ~/Downloads/* for both accounts on this machine and I have emptied the Trash many times… and finally—finally!—there is enough free space to start the upgrade including a media backup.
Curious, however, I wanted to know where stupid iTunes is saving the backup to. Well, theoretically lsof will “list open files”. Let’s see:
Pyrobombus:~ alex$ lsof|grep -i backup AppleMobi 895 alex txt REG 14,2 499200 11536761 /System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/AppleMobileDeviceHelper.app/Contents/Resources/AppleMobileBackup AppleMobi 895 alex 7w REG 14,5 2097152 3720396 /Volumes/Extern/MobileSync/Backup/346b7073bd3e141e94284c421ed96dc724f88ff9/Snapshot/8277946022662205f468e8e329528bfa8b0c9fbf.upload
I noticed that you might have to call this repeatedly because at particular points in time the process might not be writing backup files.
Anyway. What does this tell me? iTunes checks the available disk space on the default drive where it thinks the backup will be created, even though I moved the directory to an external disk. 
Well, I was hoping this fixed the stupid problem, but not so:
Now what? Retry? I wonder what that means, exactly: Loss of all in-app purchases? What if the app is no longer supported—are the in-app purchases that I am unable to download lost forever?
Another question poses itself: Do I need a backup as part of this upgrade process? When I sync the iPad before doing the upgrade, I see that it is doing a backup. When I check iTunes Preferences → Advanced → Devices → I see “Alex Schröder’s iPad – Today 11:20” (not quite what I expected but close enough). Perhaps I can just upgrade the iPad and then restore this backup?
Googling for this error leads me to a thread where a user is saying “I had my MobileSync folder symlinked…” This is exactly what I did to avoid the first error. Needless to say, this makes me very unhappy.
What I’ll try next is deleting the symlink without moving the backup data back to the main drive and upgrade. We’ll see about the various backups after the upgrade. Perhaps the 31.30 GB required are just a temporary thing. I sure hope so.
It worked! When I came back this morning I saw Syncing Photos to “Alex Schröder’s iPad (Step 6/6)—Copying photo 16600 of 19540” 