Remove the hard drive from your MacBook Air
Use this guide to upgrade or replace the solid-state drive in a MacBook Pro 13” Early 2015. This MacBook Pro uses a proprietary storage drive connector, and is therefore not compatible with common M. 2 drives without the use of an adapter. Before you perform this repair, if at all possible, back up your existing SSD.
Because the Mac Studio has physically removable SSDs, removing and replacing them should work as it does in a PC. Apple implemented some kind of "software block" to prevent the Mac Studio from booting after its storage has been replaced, as evidenced by the power LED's blinking amber "SOS" pattern.
Apple's new Mac Studio was revealed to have an interesting secret over the weekend: it has removable SSD storage.
How to upgrade your MacBook Air with an SSD
Install an SSD in your MacBook Pro
Apple price $999 plus tax. Not a preferred option. Price of SSD and Cable $83 (Amazon Prime) delivered in about 3 days.
You need a lot of storage First, Apple's PCIe SSDs will be significantly faster than an external HDD. That may not be a problem if you use the HDD to store files you rarely access, but if you frequently use it, you may feel the slowdown.
Due to this, the proper answer for MacBook Air expand storage is to upgrade MacBook Air SSD to a larger one. That is, you can replace MacBook Air SSD with a new and larger one especially when you want to save more data on your MacBook Air and experience the smooth running speed.
Apple has determined that a limited number of 128GB and 256GB solid-state drives (SSD) used in 13-inch MacBook Pro13-inch MacBook ProApple 60W MagSafe 2 Power Adapter (MacBook Pro with 13-inch Retina display) (non Touch Bar) units have an issue that may result in data loss and failure of the drive. 13-inch MacBook Pro units with affected drives were sold between June 2017 and June 2018.
Use Format: Mac OS Extended (Journaled) for hard drives, and Format: APFS for SSD. Always use GUID Partition Map for macOS. It is best to first choose View ==> Show All Devices so that both drives and volumes appear in the list at left (if not showing, choose View => Show Sidebar).
The MacBook Pro only have a single drive slot, so you can't add a second SSD. Your only option is to replace the existing SSD with a bigger one. Another alternative is to use an external storage device.