It's not the NAS fault usually.

I've switched to an external HD always plugged on my "docking station" as Time Machine backup for this reason. Although it's annoying to remember to unmount the HD (and check if a Time Machine backup is in progress) before disconnecting the laptop :/

Maybe the most sketchy thing is that I have an ancient SSD stuck in there as cache. I could try removing it, though its smart output also looks good.

Next sketchiest is that it's using btrfs, but other people tell me this is trustworthy. /shruggie

The error is rare enough that I'll never really know it has gone away permanently or what fixed it if it has, because I'm never going to go for say a year without changing more than one variable. For example, I'm not going to stop doing macOS or Synology updates for that long.

SMART will tell you that your disk is failing, not that it’s not failing.

Unhappy smart = unhappy disk but happy smart /= happy disk.

Another poster suggested this happens with AFP, which would likely mean a macOS defect. I'm using AFP. I think I'll switch to SMB...

[1] https://www.backblaze.com/blog/what-smart-stats-indicate-har...

I wouldn't put too much trust on SMART[0]: since yesterday I have a disk in a raid 5 array making loud clicking noises, mdadm says everything is fine and all disks report perfect SMART data.

[0] I trust SMART when it says it's not OK though

When the firmware detects this, it tries to reset the head, usually by parking it for a moment and then unparking it. Most of the time this solves the issue.

This inherently doesn't cause data loss or corruption, just high latency when using the disk. It can lead to an increased number of high fly writes as the head is unparked and is usually a bit too high for a bit until it finds it's air cushion again.

The usual suspects in your smart data are "Recalibration Retries", "Seek Error Rate", "Head Stability" (if WD), "High-Fly Writes", "GMR Head Amplitude" and "Head Flying Hours".

All of these generally don't cause runtime issues with the drive, but a clicking sound or an increase in these numbers means the mechanical assembly of the HDD, while still in spec and good enough to operate the HDD almost normaly, is degraded.

Of course, your head may crash down on the platter the next recelibration try, the clicking causes a lot of head movement and parking, so it also causes a lot of wear. Drives which are clicking age a lot faster and in reverse, clicking is an easy indicator the drive is beginning the end of it's useful lifecycle.

The SMART data changed, I think it was the seek error rate that went through the roof.