Got this today:

Disk health
S.M.A.R.T. status: BAD
Reallocated sectors count: 1,872 (threshold exceeded)
Back up your data and replace the disk as soon as possible.
The NAS drive holds a lot of my backups and this is the first SMART warning I've ever seen. Percentage Used went from zero to a small number. Should I replace it right away, or is it normal for a few sectors to go bad? I've read conflicting advice, so I'd like to hear from people who actually watched a drive with a warning like this. Did it fail fast, or did it last for years?