Why is this one drive so hyperactive?

Hi,

I wonder, why this one disc is so active all the time...
As you can see in the attached screenshot, my ada7 is busy all the time. All the other drives show no activity.
ada7 is a single disc pool, the other (visible) drives are together in one pool. Another disc also is a single pool, but shows no activity. All pools are shared over SMB. Settings are all the same for each drive.

Why is this one drive so hyperactive?

Thanks for any hints.
(I'm new to TrueNAS, please be gentle :])