I want detaild for Any Hard disk fail Replacement - Expected Rebuild Time ?
It's hard to give a definitive answer. Users can access the files while the resync is going on, and the more they do that the longer it will take. Of course it would also depend on the disks.
Perhaps other posters who've done disk replacement recently can share their times.
Even which model NAS it's on is going to affect the time. On a similar NAS, I added a 6th 4TB drive to a RAID5 and it took just under a day to re-sync. That's the closest thing I have.
You can do an MDADM check and get a good idea of how long it will take, perhaps initiated over a weekend or some time when access will be the lowest. You have to do that from SSH with the command
/usr/share/mdadm/checkarray -all.
Note that your RAID5 (I assume) and spare is not the best for data protection, as your data will be vulnerable to a second drive failure during the re-sync to bring the spare into the array. RAID6 gives you dual failure protection with the same number of drives.
If you have multiple arrays and don't want to dedicate a second drive to redundnacy on each, then a single global spare offers some benefit (you don't have to be present to swap/add a drive to start the re-sync). But with a single volume, I don't see the benefit of RAID5 and a spare over RAID6 other than RAID5 is a bit faster (including re-sync for the same size array).
I have a 12-bay NAS with two RAID5 arrays and another with a single RAID6, and my access to both is about the same -- my network connection is the bottleneck. That might be a bit different with multiple users accessing simultaneously.
