The 11" Early 2015 MacBook Air has an internal SSD. Apple shipped it with 128, 256, and 512 GB SSDs, and you can get replacement SSDs that hold as much as 2 TB from Other World Computing.
If you are coming from an older laptop (Windows or Mac) that booted from a mechanical hard drive, having a SSD for the first time should make a big difference in startup and application launch times.
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It is a good idea to keep at least 10% – 15% of the space on your startup drive free. If your MacBook Air has only 128 GB of SSD space, and you find yourself starting to fill it, you may want to consider getting a portable external drive, or upgrading the internal SSD. (I recently bought an external 1 TB SSD which was about the size of a credit card, and about as thick as an iPhone, for ~$60. Not the fastest SSD in the world, but I'm happy with it so far.)
(For internal SSD upgrades, see:)
OWC Solid State Drives for MacBook Air 11" & 13" (Mid 2013 - Mid 2017)
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The difference between an external "PC" drive and an external "Mac" drive is often that
- The "PC" drive comes pre-formatted with NTFS
- The "Mac" drive comes pre-formatted with HFS+ or APFS
It takes very little time to reformat a "PC" drive as a "Mac" one using Disk Utility. So if you're shopping for drives, you can buy whichever type is on the best sale.
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Steer clear of most "cleaner", "optimizer", etc. apps – these may do more harm than good.