Wow 50TB of external storage

You have a lot terabytes of data that is not important and can be replaced if damage/missing, and some important data that irreplaceable? If so then I suggest use 2 HDD for redundancy/RAID for the important data while the rest is just a bunch of disk, no redundancy or backup. This way it doesn't cost so much for those not important data.

As for the NAS itself, you can go pre built like Synology or built yourself. Usually if need until 100TB it would be cost effective to built yourself, but need source each component for NAS use by yourself, learn how to install the OS and setup everything. Need some technical skills. But since most of the data is not in RAID, you can go 6 or 8 bays pre built NAS, 2 bays in RAID-1 for important data, while 4/6 bays each as own volume, this way you can use it as hot swappable, take out currently not need HDD and put in another HDD that you want to access. This way no need to get much more expensive plenty bays NAS while still can have 100TB now and expand in future.

For HDD, price per TB, now 8TB is the most lowest. Toshiba N300 is less than RM900 while Seagate Ironwolf is less than RM1100. Not recommend to get WD now because it might be SMR drive.
Depend on your external storage HDD in it, some if it using normal HDD inside, you can take out the HDD and use on NAS.

For NAS, if you built yourself it might cost around RM2000+ if everything is new. If prebuilt, 6 bay NAS I like DS1621+ hardware, price can get around RM3600. This comparison show how much more powerful the hardware is. If 8 bays NAS, DS1821+ still not yet selling in Malaysia, DS1819+ (around RM4500) hardware also not bad but random read/write not as fast as DS1621+.
DIY NAS pros is cheaper and can customized everything. Cons is need technical skills and time.
Prebuilt NAS pros is not much technical skills need to setup and maintain, everything is mostly automated. Cons is the hardware is more expensive, cannot upgrade.

How long to take for you to expand the size from 0 to 100tb?

And are you willing to spend on the initial cost?

That determines the type of NAS to get.

Important as in the data cannot be damage or missing? Then you will need store in system which have checksum to maintain it's integrity such as RAID-5 or BTRFS. You also need to have backup of the data. If use prebuilt NAS get at least 8 bays. 8TB HDD won't cut it anymore. 12TB HDD x 8 you will have 84TB usable space if use SHR-1 or RAID-5, this method 1 HDD failure you still can replace it and get back all data. But too many HDD means chance of more than 1 HDD failing is higher, use SH-2 or RAID-6 can have 2 HDD failure and still get back all data, but this RAID type only 72TB usable space. You also need to get additional HDD as backup copies. So maybe around 12TB x 8 for storage and 8TB x 7 for backup.

If it's just video where file integrity is not really important, example if some bit-rot, the video will have glitch on some time frame, it's still acceptable. Then you can just use each HDD as own volume without any RAID, then get same total capacity of HDD for backup so that when main HDD fail you still have another backup.

If the data is super important, best practice is to have 3-2-1 backup rule. Have 3 copies of the same data, at least have 2 different storage media (another in dvd, bluray, tape drive) of the 3 copies, and store 1 copy offsite (in the cloud or at other home/location)

Wouldn't it be better to suggest him 12 or 16 bay instead as he doesn't seem the type to have a limited budget
This way, he can actually achieve the 100TB he wanted

With the storage requirement, you may want to consider unraid setup with 2 parity drives?

That is fast phase.

So you have to start with 50tb initial.

The best way I think is 12bay for the start.

7 x 16tb hdd with SHR Raid 5 to begin.

That would give you 6 x 16tb = 96tb and 1 redundacy. 96tb - 50tb =46tb free. This should last you for 2yrs+.

And each yrs, you can add 1pcs hdd or 2 to increase the size. Till max 176tb.

176tb max on 12bay can last you for 8yrs+ and more.

The 12bay is not limit yet. If use Synology.

12bay NAS can expand the storage to another 24bay. (12 bay expansion x 2 only storage without CPU)

That is the cheapest cost for easy setup and maintain I guess.

Really similar like having platinum credit card?

Usable storage if SHR-1/RAID-5 is 144TB, but since 10 HDD pool, having 1 HDD failure protection is kinda risky. SHR-2/RAID-6 would be better.
Andrewtst
If buy few HDD first and in future keep adding HDD with different size, it would be better to use SHR since it allow to maximized the pool according to HDD space
ozak
It seems there's no way in DiskStation to set a HDD as removable drive? I doesn't find if there's such option.

It's a RAID type, develop by Synology and only usable for their NAS. Classic RAID type size limited by the smallest HDD in the pool. SHR can maximized the storage without being limited by the smallest HDD. Can read here to understand more.
https://www.synology.com/en-global/knowledg...Hybrid_RAID_SHR