1720-SMART Hard Drive detects imminent failure (failing Attribute :5) + ProBook 4520s

SOURCE: I get a 1720 Smart Hard drive detects imminent

It means that the Seagate hard drive is near death. Backup all your data immediately, create a recovery cd starting from the recovery partition situated on your disk (if you haven't done this before or you weren't provided one when you bought it), replace the hard drive and reinstall.

SOURCE: I get a 1720 smart

This warningmeans the hard disk will fail sometime in the near future. You need to back upyour data regularly before the hard disk fails or you will lose your data.
When the hard disk fails, you need to replace with a new one and installWindows on the new hard drive to get your laptop working again.

SOURCE: 1720-smart hard drive detects imminent failure (failing attribute : 1)

This is a warning that your hard drive will fail in the near future. It is wise to backup your data frequiently so you won't lose your data when the hard drive fails.
The faults your hard drive is experiencing will cause the computer to slow down as it tries to read and write data to your hard drive.
There is nothing you can do to fix the hard drive except to replace it when it fails.