The amount of data on my iPhone keeps growing and growing (I regularly use a 128GB model) - while my trusted old MacBook Air from 2011 keeps loosing and loosing free SSD storage space. Also in terms of strategies to backup an iPhone, for the general public there are two strategies:

• You could either go with Apple's iCloud service. The name is the game - it is a cloud based service which gives you all kinds of stuff, for example space for syncing your photos across your i-devices. And it also gives you a wireless, cloud based backup service.Basically the app user data and phone settings will be backed up to Apple's servers. Apple gives you 5GB of free storage space and offers additional paid plans. Recently a new pricing was announced - and there is now a new top tier plan which provides 2TB of storage for $19.99 per month ($239,88 per year).
• Or you could go with the traditional iTunes backup. Meaning you would hook up your iPhone with your PC or Mac (either via lightning cable or via Wi-Fi) and backup using the iTunes software. There are a couple of things to keep in mind when considering this method. Basically the backups will be stored locally on the respective PC or Mac (in kind-of the user library folder of iTunes - more details on folders later on). So you have to make sure there is enough space on the computer and if the computer breaks your backup is gone as well. One thing I regularly ran into: Your computer's iTunes software has to be fairly up to date. Usually with iOS updates come updated versions of iTunes. So quickly powering up an old PC with lots of TB storage space usually involves downloading and installing iTunes updates.

I personally believe for my use case iCloud is to expensive (and in some scenaries too dependant on my internet speed at home). So my backup method of choice used to be the iTunes variant - until that time that my MacBook is running out of space.So I was looking for a decent way to back up my iPhone 6s Plus to an external hard drive.For Macs running iTunes there seems to be an elegant solution (I am not so familiar with symbolic links on Windows - however it seems that a similar solution  could work on Windows: Guide at TechinOurLife.)My internet search surfaced this very useful guide: https://discussions.apple.com/thread/3421834?tstart=0

1. Pick an external hard drive and make sure there is enough space. Create a folder named "Backup" on your external drive. This can be done via Terminal with this command:

   1. "mkdir /Volumes/<ExternalDriveName>/Backup"

2. Rename the local / standard iTunes backup folder on your Mac. Again the Terminal command:

   1. "mv ~/Library/Application\Support/MobileSync/Backup~/Library/Application\ Support/MobileSync/Backup2"

   2. (this will keep the old local backups around - so make sure to clean this folder up if you need space / if you no longer need older backups)
3. Create a symbolic link (these work well on OS X - I use them for a swift dropbox integration). This will look like the local iTunes Backup directory but it will point the computer to the external folder drive (specifically the folder we created in 1.). Again the Terminal command:

   1. "ln -s /Volumes/<EternalDriveName>/Backup~/Library/Application\ Support/MobileSync/"

4. Launch iTunes and initiate the backup
5. done

Please note: when the external disk is not connected the iTunes backup process will result in an error.