I recently pulled a SATA hard drive out of a Linux box that I wanted to grab some files off of. I only had my Mac on hand, and I had a USB 3.0 to SATA hard drive adapter at the ready.
But when I plugged in the hard drive, macOS said it couldn't recognize the disk.
Makes sense, because macOS includes support for Apple's filesystems, not Linux (or even NTFS, Windows' preferred filesystem). There are commercial solutions you can buy, like Paragon Software's extFS for Mac, but that costs $39, and I don't want to deal with the licensing issues that may exist there if I just want to grab a few files off one hard drive.
Luckily, there are some open source libraries that allow at least read only access to ext4-formatted disks on macOS. Let's install them and use them to mount the drive: