Erase and Install on Air?

[spacewolf]

It's been a long time since I've done an erase and install and I wanted to ask you guys some questions about it.

I'm wanting to erase my Mac completely and reinstall Yosemite, so I have a bootable USB (which seems to take a long time to boot). After booting up I go to disk utility and theres a number of options which is where it gets a little confusing.

Previously I've been able to erase DISKS and VOLUMES. Why can I only erase the VOLUME of Macintosh HD?

There is the Internal SSD and the USB and there are options below also with yellow icons, can the options below be erased at all? I realise they are for recovery but I thought the system would reinstall these after I erase the disk and reinstall yosemite.

Erase and install is not what it used to be on my MacBook Pro, can anyone shed a little light on this because I feel like I'm not erasing everything and I don't like it.

 

[Dweez]

Have you tried the Partition option within Disk Utility? Select the target Disk, then select the Partition tab to see what your options are.

 

[spacewolf]

Ive never partitioned the SSD and don't want to play around with that. Is there a particular reason I'm not able to erase it though?

 

[Dweez]

Disk Utility -> Select Drive -> Partition -> 1 Partition -> Mac OS Extended -> Apply. That's effectively erasing the drive and preparing it to reinstall the OS.

 

[spacewolf]

thanks for the info. I'm still looking to find out why I can't erase the DISK and only the VOLUME though. I was able to do this in previous versions of OS X and before I had the MacBook Air. Maybe its an SSD thing?

 

[motrek]

thanks for the info. I'm still looking to find out why I can't erase the DISK and only the VOLUME though. I was able to do this in previous versions of OS X and before I had the MacBook Air. Maybe its an SSD thing?

No. I can't remember when/if they changed the design but the problem is that disks require partitions. Even if it's only 1 partition that takes up the whole disk, that's where you put your data. Not your disk directly.

So if your disk already only has 1 partition and it takes up all the space, you can erase that and you'll be set. Or you can repartition the disk. But you can't erase the disk, that doesn't make sense technically.

 

[spacewolf]

I know there technically has to be one partition, but previously I was able to erase the DISK and the VOLUME would dissapear and re-create itself. This is no longer an option.

 

[motrek]

I know there technically has to be one partition, but previously I was able to erase the DISK and the VOLUME would dissapear and re-create itself. This is no longer an option.

I guess they changed the software to better reflect reality.

 

[spacewolf]

How so? What if one had multiple partitions? Erasing the disk would delete them all easily enough. Seems a little bit odd to me.

I noticed that the DISK says it has 18MB free - whats that all about? Why would that be so?

 

[motrek]

How so? What if one had multiple partitions? Erasing the disk would delete them all easily enough. Seems a little bit odd to me.

I noticed that the DISK says it has 18MB free - whats that all about? Why would that be so?

The UI allows you to edit the partitions, so if you had 2 partitions, you can very easily click on the disk, click on the Partition tab, and then switch it to 1 partition.

If you were allowed to erase the disk and thus the partition table, then you would end up with 0 partitions and the disk would be unusable. This is never what anybody wants.

 

[spacewolf]

Well I guess I was used to doing it the old way that Apple let me for so many years. Still though that 18MB free is puzzling me.

 

[motrek]

Well I guess I was used to doing it the old way that Apple let me for so many years. Still though that 18MB free is puzzling me.

Dunno where you're seeing 18MB free but if you repartition maybe it will go away?

 

[spacewolf]

Whoops - I meant available...

My DISK is showing as 250 GB capacity, 18 MB available and 250 GB used.

My VOLUME is showing as 249 GB capacity, 233 GB available and 16 GB used.

Having never re-partitioned the disk I'm puzzled by the 18 MB

 

[Dweez]

I don't claim to know how the HFS driver works, but perhaps the 18 meg free is for the OS to do housekeeping on the drive?

 

[spacewolf]

that seems feasible at this hour.

 

[motrek]

Whoops - I meant available...

My DISK is showing as 250 GB capacity, 18 MB available and 250 GB used.

My VOLUME is showing as 249 GB capacity, 233 GB available and 16 GB used.

Having never re-partitioned the disk I'm puzzled by the 18 MB

My disks show either 4k or 0 space as available. Dunno how you got 18 MB. It's small enough (compared to the total disk size) that you might as well not worry about but if you're going to erase the drive anyway you might as well repartition it to see if that free space gets reallocated.

 

[Weaselboy]

How so? What if one had multiple partitions? Erasing the disk would delete them all easily enough. Seems a little bit odd to me.

I noticed that the DISK says it has 18MB free - whats that all about? Why would that be so?

Here is what happened... the Yosemite install converted your drive to a core storage volume and that is why you cannot manipulate the volumes/partitions normally.

Boot from your installer again and once in the recovery screen go to the Utilities menu and start Terminal then enter the command below. I am assuming here your main volume is named the default Macintosh HD. Make sure you include the quotes.

diskutil cs delete "Macintosh HD"

That will nuke the main volume and leave you a blank disk. Now quit Terminal then start Disk Util and go to the erase tab and select the drive itself at the top of the left column and format to Mac OS Extended (Journaled). Then quit DU and go ahead with your new install.