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I bought a 13inch macbook air 2011 edition with 128 GB capacit5y, I want to increase it to 256 as I thought Apple usually charges a premium on flash memory. On ifixit site it was said during teardown that it is possible to increase SSD but not RAM. But can you suggest hopw and where , and if it will be cheaper than Apple ? If not I might as well return 128GB and buy 256 gb one if the price diff is not that much.

It can be done, but the problem with getting a drive is the interface is nonstandard. You will need to find an Apple SSD or go with compatible 3rd party options. While you can probably find an Apple drive, you’ll probably find that the SSD from OWC is a better option.