An email indicating "DISK error notice" was received.

Email notification  sends the following, as an example.

DISK error notification
HDD error has been encountered.
error location: disk 2
READ sector：956062896 count：6
Disk Read error was detected. RAID drive error will be automatically recovered.
Although an immediate hardware failure is not expected, a regular data backup is recommended.

This email indicates that a read error occurred in case of drive access at sector 956062896 of Drive No.2
As this does not mean a hard drive failure, an immediate hard drive replacement is not required.
A frequent reception of such an email suggests that a replacement of the hard drive should be prepared.

When an increased read error, the following error message appears.

"HDx Error E15 Many Bad Sector"

This indicates a replacement of the hard drive is required.

When a hard drive fails, an error message appears on the LCD and the failed drive’s status LED will be glowing a steady red, which indicates that the hard drive should be replaced.