From the ever so helpful people at Backblaze:

For the last few years we’ve used the following five SMART stats as a means of helping determine if a drive is going to fail.

Attribute	Description
SMART 5	Reallocated Sectors Count
SMART 187	Reported Uncorrectable Errors
SMART 188	Command Timeout
SMART 197	Current Pending Sector Count
SMART 198	Uncorrectable Sector Count

When the RAW value for one of these five attributes is greater than zero, we have a reason to investigate. We also monitor RAID array status, Backblaze Vault array status and other Backblaze internal logs to identify potential drive problems. These tools generally only report exceptions, so on any given day the number of investigations is manageable even though we have nearly 70,000 drives.