Replace bad HDD of Synology NAS DS412+ on DSM 5

TL;DR

Replacing 1.5 TB drive with 4 TB took 10 hours for 4.5 TB used on 5.5 TB SHR array

information collection
- max supported storage: 16 TB (4 TB in each 4 bays)
- latest DSM version: 6.0 is out (I postponed to update after replacing HDD)
- RAID calculation: existing 1.5+2+2+2 = 5.5 usable to 2+2+2+4 = 6TB usable (SHR1)
- determining HDD brand: WD Green & Seagate Barrucada available and I chose Seagate as WD green’s head parking cause problem for NAS
- SMART attribute: I have non-zero nubmers under Reallocated Sector Count & Drive Reconnection Count & and everyone suggested Reallocated Sector Count should be zero (and if non-zero, backup data and replace ASAP)
getting compatible disk model – check this page before buying
deciding which HDD size to buy (using RAID tools) – this calculator can show how much disk space will be used or unused (make sure to read instruction before you buy HDD)
reading instruction – Synology KB has complete instruction on how to perform. Left it open to read while you are performing the steps.
buying – bash out the money, check physical defect and warranty period
physically replacing – note which bay to eject, shutdown, eject bay, replace HDD, insert bay, restart, then disable beep (if beeping)
repairing – inserted HDD is uninitialized. Initialize and repair the volume group.
expanding – depending on the size of newly inserted HDD, you might need to expand the volume. Mine can expand another 500GB. It took less than 1 minute to expand.
pricing calculation 44 MMK/GB

SMART Value means normalized Value and it should be greater than Threshold number
For reallocated sector count, not only the number but also the rate of increase in number is important.
I had 2 (out of 4) HDD having non-zero reallocated sector count.
1. 1.5TB with lower number but much higher power on hour
2. 2TB with much higher (4 digit within 4 months)
Contrary to theory I learnt, I changed the 1.5 TB with 4TB. Now I am running with those 4 digit sector count.
SHR1 can accept one drive failure
SHR2 can accept two drive failure
make sure you use Uninterruptible Power Supply during repairing