External drives · Speed, cables and hardware
Why does my external hard drive spin up when I'm not using it?
Short answer: Something is using it — typically Spotlight indexing, Time Machine, photo or music apps, or backup tools, and Storage Studio checks its drive from time to time too. Activity Monitor's Disk tab shows what's reading it.
Something on the Mac is reading or writing it. The usual visitors are Spotlight indexing new files, Time Machine backing up to it, photo or music apps checking their libraries, and backup or sync apps. Storage Studio also checks on its drive from time to time.
To find out what, open Activity Monitor → Disk. If indexing is the cause and you don't search that drive, you can exclude it from Spotlight — see excluding the fast drive from Spotlight. Brief activity is harmless.