With my five and a half year old MacBook Pro in the shop for the third time, it was time to revive my mid-2012 Mac Book Air (MBA) which was on hiatus last semester. Having maxed its RAM at purchase to 8GB, and having changed the battery last year, it is in reasonable shape.
The internal hard disk is a fast SSD but small at 500GB. So my most immediate files are on the cloud in Dropbox, GDrive or iCloud (and at work, nBox or the office server). I’ll survive with some discomfort and that will spur me to replace the faulty 1TB SSD I got from from OWC (I bought the kit below). Once I get my replacement, the MBA will be ready for some heavy lifting.
Back up happens by virtue of working on cloud drives, and a background programme, CrashPlan Pro. And just in case, for the most current presentation, I have an emergency 200gb microSD card.
This mid-2012 Mac Book Air has a few ports: one thunderbolt, two USB3 and an SD card port. And I pack thunderbolt adaptors for VGA, HDMI and ethernet with it so that makes it functional in most places.
Some archives are residing on external 4TB and 5TB drives and must be backed up to Crash Plan Pro. Their subscription includes unlimited backups on multiple devices including external drives, so less e-waste at the user’s end eventually. I tried retrieval off files and its not bad. Important when pulling out relevant photos or videos from a field site ages ago!
If you have old Mac laptop and want a second life with it, check your model (Apple venue > About), and then examine your options at the OWC upgrades page. If your unit is DIY-capable, there are videos to guide you through the process. With the SSD HD and battery upgrades (and some repairs), my laptops are soldiering on in their sixth year and the iMac is doing well in it’s ninth year.
Otterman speaks... 
These upgrades certainly extend the useful life of computers.