If a hard drive has S.M.A.R.T. warnings, a color mark will appear on its left-top corner. Drugging the cursor over the drive will show a tooltip explaining that warning.
Warnings will also appear in confirming e-mails for scheduled actions.
! ST3000DM001-9YN166 (2.72TB #1): Health Status: CAUTION
 Reallocated Sectors Count: 64736
S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) is a technology widely-used in hard drives and solid-state devices that monitors their reliability conditions to predict possible hardware failures.