So I had a 3 year old WD20EARS redball on me last week. I had a spare drive so I swapped it out and ran tests outside the array. Both the short and long SMART tests passed showing 1 pending sector. I ran a 3 cycle preclear and had no further issues.

 

How risky would it be at this point to reintroduce this drive to my array?

If it passed the extended SMART test with 1 pending sector then at this time that is a false positive, disk is good for now, now in regards to your question, depends on how adventurous you are with your disks, in my experience a disk that failed before has a much higher probability of failing again, but it can also work for years with no issues, so depending if you have single or dual parity and/or backups you have to decide the level of risk your comfortable with.

Keep in mind that unraid uses all disks to recover a failed drive, so you are risking the data on all your disks when you knowingly use a flaky drive.