I have been mystified by the gradual decline in free hard disk space on my Macbook Pro. It accelerated over recent times, around the time I upgraded to El Capitan, and exacerbated by a temporary failure to my primary Time Machine device, and my initial instinct was to blame the new Operating System. It turns out, I was wrong. The answer was a default characteristic of how my Time Machine Backups work. Here’s what I found out.
The details are described here in this Apple Support Article, but the consequences are perhaps a little less clear. It turns out that Time Machine Backups take “Local Snapshots” even when you’re not connected to the Time Machine backup device you’re backing up to, if your machine is a portable computer. And these are stored on the primary storage device of your machine.
My own experience was an “unexplained” but relatively constant drop in hard disk space, which I now understand to have been taken up by these Time Machine local snapshots. My personal profile of use of my MacBook Pro is 80% – 90% of the time permanently connected to my Time Machine device, and 10% – 20% of the time mobile or disconnected, so my concern was that I was unconvinced that these local snapshots added huge value.
This concern was increased when I found after trying to clear up space recently, a huge amount of space tied up in a directory related to these local snapshots – over 100 gigabytes – in a directory called /.mobilebackups.trash. All my research indicated that this related to local snapshots, and recently deleted files and the like, but given it was over 100 gigabytes in size, this concerned me. My MacBook Pro has a 500 gb SSD, but even so, for a single (silently configured) feature to be consuming one-fifth of my hard disk space didn’t sit well.
There are some passionate discussions on various blogs in favour and against disabling these Time Machine Local Snapshots – as an example see this one – but on balance, given the nature of my use of my MacBook Pro – it’s almost always connected to its Time Machine device in the evenings for example – I’ve opted to disable Local Snapshots, as per this article, and freed up over 100 gigabytes of space. I’ll report back on any adverse outcomes, but so far, all is behaving just fine, and I’m now back to what I consider a much more comfortable amount of free space on my 500 gb SSD.
By the way, performing the steps in the article above to disable local snapshots, freed up the space directly, no further steps were required in my case, and I suspect for those for whom further steps were required, perhaps some disk corruption was present.