Help! OS X Base System only disk available

Just for a little background:

• What is the 'Mac OS X Base System' disk image on my 2011 MacBook Air? - AskDifferent

tl:dr

The Mac OS X Base System is mounted only in Recovery Mode and is used to install macOS without a CD.

Just adding the MacIntosh HD volume would not be the best way to get your Mac back up and running again.

At this point, I would suggest that your use Recovery Mode to first, completely erase your Mac's system drive, and then, reinstall macOS. Your 2012 MacBook Air should be able to be upgraded to macOS Catalina 10.15.7

Ref:

• Use macOS Recovery on an Intel-based Mac - Apple Support
• If you can't start up from macOS Recovery - Apple Support

I think your Mac might only have 51 GB of total space because you added the extra "Macintosh HD" partition as shown in your Disk Utility screenshot.

To get rid of the extra partition, and rename your startup volume to something more normal:

1. Start up your Mac normally and log in.
2. Open Disk Utility, located in Applications -> Utilities.
3. In Disk Utility, select View -> Show All Devices.
4. Select the Apple SSD entry in the left sidebar and choose Partition.
5. If you're asked to add an APFS volume, choose Partition instead.
6. Select the Macintosh HD partition (should be near the top left side of the pie chart), then click the minus (-) button to remove it. Click Apply.
7. Your Mac might become unresponsive for a bit as it resizes the startup container. Just leave your Mac be and wait for the operation to complete.

Now, in Disk Utility, your current startup volume is named "OS X Base System". This can be confusing for us and others since that's also the name of some older macOS Recovery images. To rename it to something more normal:

1. Select the OS X Base System volume (indented below the Apple SSD entry) in the left sidebar.
2. In the main pane on the right, click on the bolded name. It should let you rename it.
3. Rename the volume to "Macintosh HD".

I took a look at the specs for your Mac model and discovered that it's possible that your Mac only came with 64 GB of internal storage. This is very small compared to modern standards and doesn't give you much space to work with. Modern copies of macOS take up around 15 GB alone, not including other system caches and data.

To try to free some space taken up by internal app data and some other things, take a look at this user tip: What is “Other” storage on a Mac, and how… - Apple Community

[Edited by Moderator]

Hi Nadziejkax,

Glad to hear you successfully made it into macOS Recovery!

Are you able to start up from the "OS X Base System" container? Try clicking the Apple logo in the top left corner, and choose Startup Disk. Does it appear there?

If it doesn't appear there, or if you want to completely erase your Mac and start over, complete the steps below:

In Disk Utility, select the Apple SSD in the left sidebar. Then, choose Erase, select these options, and click Erase:

• Name: Macintosh HD
• Format: Mac OS Extended (Journaled)
• Scheme: GUID Partition Table

When the erase is complete, quit Disk Utility and choose Install macOS from the macOS Utilities window. Connect to Wi-Fi and complete the onscreen instructions to reinstall macOS.

I'm going on your comment that "I don’t mind resetting the drive or anything just don’t know how to."

As such, I provided you with support article links on how to go about doing so.