You should monitor awhile before you RMA your drive. Just because you do a RMA doesn't mean you will get a "new" drive, mostly refurbished. Furthermore your warranty is for 3-5 years depending on the model of drive. The NAS can take a hit of bad sectors on hdd that is why Synology reserved 10% off the total drive space for this, to relocate bad sectors. If your bad sectors are not growing you are better off with your current drive. This is only my opinion after RMA'ing many drives before. I have received dud drives before after RMA too.

Indeed monitors for few days... from hundreds to over 10k bad sectors already plus another sync errors appear from Synology lastnight

so decided to take it out and fill-up rma request

Better RMA it.
Recently this happen to me, 1 of the HDD in my Synology NAS has bad sector (<10) but it didn't grow so I decide don't replace it first.
Few weeks later another HDD failed and cause volume crash (RAID 5), luckily my Synology is in HA mode and auto failover to passive NAS.

I guess many possibilities could have happen, even if you have changed the hdd and another drive failed volume crashed would have still happened. If your data is crucial requiring HA better SHR-2 or Raid 6. Drives failing is common nowadays is really depends on luck. But in his case over 10k bad sectors that is really bad.

I have a 8-bay Synology NAS with 7 HDD on SHR-2 and slot 2 HDD has failed. While wait for RMA HDD, can I add 8th HDD to expand the storage capacity?

if I get you right, currently you have 7 drives on shr-2 and one drive died. when you pull that drive out to rma and put another drive in say the last bay it will repair itself. the sata positioning does not matter, the OS is built to detect the drive order again, so I don't think it will expand the storage. that's if I got your scenario correctly.

Is this how it work? Where can I find any documentation/information?

Can start at here
https://kb.synology.com/en-global/DSM/help/..._disk?version=7

Yup indeed in Raid-6 now feels comfortable waiting for the RMA rather in Raid-5 last time

i have a i3 (8th gen) intel cpu
16GB 3200 DDR4
120GB & 250GB SATA SSD

Wondering what i can repurpose it for..

wat an unfortunate 2nd RMA received after 15 days was DOA

now do another RMA well should I wait 3rd RMA, leave the NAS in degraded condition another few weeks or bought new unit

So unreliable RMA drive, died that early.
Yours is RAID-6 right? Maybe get another drive and after full scan put as cold spare. Can't trust the RMA drive as cold spare.

What brand and model of hard drives you installed in your Synology?

Indeed Yup on Raid-6 thanks for advice, will get 1 spare.. maybe pro unit

But we can still use the degraded storage is it while waiting replacement

Yes, can still use. Just the risk is there, if another drive fail then whole array gone.

noted.. will get new unit 😆

There's people posting that DSM version 7.0.1-42218 auto update failed to install. This version was release last month on 13th but later revised on 21st, might be revised again. I also just receive notification for the update today, better don't want install it for now.

owh not received any update so far will keep refresh

All hard drives are Seagate Ironwolf 8TB.

hoorey my RMA arrived... after total of 26 days

Doing SMART check before initialized & repair pools