Sometimes after you connects a usb external hard drive to Mac, you notice that the hard drive keeps on spinning yet nothing shows up. It could be because of the hard drive was not ejected properly last time it was plugged in and the system is running fsck(File System Check) on the background. This can take a while especially when the hard drive has lots of data. Unfortunately, there is no indications of this happening except in Activity Monitor showing a running fsck process. This can make people believe that something broke. This has happened to me and my friends before and we all thought that we had broken hard drives. It wasn’t until I plugged my hard disk to a Windows computer and found out the disk was all right. I then started to troubleshoot the issue and found out a running fsck process in Activity Monitor when the disk is uncleanly ejected. After a while when that process finished running, the disk finally showed up in Finder.

Apple should improve the UX of this by making some sort of progress indicator for the File System Check running on the background so that people won’t get confused and thinks that they have a broken hard drive.