I was told by a mac repairman that I should leave about 10% of the disk space empty, otherwise, my SSD and/or the machine would die. Is this true? I've never heard of such a thing in my life.

He also told me I should wipe the disk and do a clean install because I used the disk close to full for some time. This also doesn't make sense to me.

2015 MBP, 16GB, 256GB

  • I suggest you read this and decide for yourself: eclecticlight.co/2022/05/18/…
    – Linc D.
    Commented Dec 29, 2023 at 21:52
  • @LincD. Most of that article is focused on how much space the system takes. The only parts of that article that are relevant is the note about virtual memory and disk snapshots.
    – Ezekiel
    Commented Dec 29, 2023 at 21:55
  • "...[T]he total free space required now comes to around 60 GB for the Data volume in a boot container, or less than 10 GB in a normal storage container. Now you can add your own allowance to ensure that you’ve got headroom to use that disk, taking it perhaps to 100 GB for a boot disk, and 25 GB or more for others. If you’re really tight for space, when using a Mac with very little internal storage, or you’re squeezing multiple systems onto an external disk, you can get away with much less, but will be at risk of not being able to update macOS or install a new version of Xcode."
    – Linc D.
    Commented Dec 29, 2023 at 22:17

2 Answers 2


In terms of recommended maximum disk usage, 80-90% is a good guideline. There are two main reasons:

a) The system will expect to be able to create swapfiles, disk snapshots, etc. and downloading things like OS updates. Without enough free space, this won't work.

b) There are also speed/performance implications for the SSD, however these should become an issue at a much lower threshold than the first point, so less relevant.

For your second point, there is absolutely no need to do this. If you're having trouble recovering free space, feel free to search or ask a new question on that topic. But if you're not having trouble then there is no need to do a clean install.


Erase install is a very good thing to do in general since it lets the file system be trimmed and all allocations optimized. It also counteracts a tendency for data to become corrupted over time. That corruption is very rare, but an erase is the only sure fire way to correct this.

You Mac will not die when you fill the disk. Some optimizations stop, then poorly programmed apps can have struggles and then eventually if you 100% run out of space and get OS errors, some files could be corrupt. Backup will save you from that corruption - but you have to know to restore files from before the filling and potential errors so you lose time.

No rule of thumb is perfect. 10% free is very good metric though.

You need 25 GB free to do an upgrade and things work best with more free in my experience. Only you can decide if you can spare that much space if you buy a Mac with 128 storage. In your case, I would recommend more than the tech to be free for best performance and lest chance you’ll get too full too quickly.

My hunch is the tech is trustworthy and responding to more than the details you posted and you need both tasks based on all the circumstances they uncovered. The two recommendations are not related in most cases, though.

  • "Erase install is a very good thing to do in general" How often one should do that? Why should he do that? Can using onyX every quarter be an alternative? Commented Dec 29, 2023 at 23:50
  • Idk if tech was honest but I know he overcharged me :d I can't think of any other relevant details. I saw the tech because macbook was showing black screen and nothing I tried worked. I might have put too much pressure on it (my head). It also went from 0C to 20C. Tech said humidity difference might have causes it. That also didn't make much sense to me because there was barely any humidity difference in places I was in Commented Dec 29, 2023 at 23:52
  • I have a very general comment. If someone tells you to do something that isn't obviously a good idea, you should ask for data to support their recommendation. "Rules of thumb" often circulate based on nothing but the fact that people keep repeating them. This principle applies to life as a whole, not just to computing. Here is my rule of thumb: the less time you spend fussing with your computer, the more time you'll have to do other things.
    – Linc D.
    Commented Dec 30, 2023 at 2:44
  • Great question @user3722246 i would always recommend someone consider an erase if they think their Mac is messed up. I would recommend it strongly before you pay someone for help. Only exception is pay someone to help you back up ( and only back up) and then try to fix it yourself before a second paid help session. Advice for a casual user is often different from someone that makes their living on a computer.
    – bmike
    Commented Dec 30, 2023 at 14:58
  • I did a clean install 2 years ago, I don't think its messed up Commented Dec 31, 2023 at 0:33

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