From what I found online it seems the max capacity is 24TB. Yet when I looked at the computability list I see drives that are larger than 6TB. Does that means that if I add say 4 x 8TB I'll have 32TB available (obviously less actual space due to RAID 5) ?
Second question, if I only replace 1 of the drives from 2TB to 6TB will I get more space once the new drive fully synced? Or do I have to add 4 drives one after the previous synched before I'll have more space?
From what I found online it seems the max capacity is 24TB.
Actually no. Netgear uses the largest available drives when they publish their datasheets for the "max capacity". The RN204 was launched in 2015. Back then 6 TB was the biggest drive, and 4x6TB gives 24 TB total.
There is no known limit on the largest drives an OS-6 NAS can take. My own RN202 is currently using 14 TB drives in jbod, so the total capacity is 28 TB. You could double that on the RN204 (or go much larger, since 22 TB drives are now on the market).
That said, the RAID sync time does go up as drive capacity increases. And at some point, the amount of memory could constrain how big a volume you can have.
Second question, if I only replace 1 of the drives from 2TB to 6TB will I get more space once the new drive fully synced? Or do I have to add 4 drives one after the previous synched before I'll have more space?
For most people, XRAID is the best option for RAID management. Note that if there is a green stripe on the XRAID control on the volume page, then XRAID is enabled (and it is enabled by default).
The capacity rule for single redundancy XRAID is "sum the disks and subtract the largest". So upgrading one disk won't increase the volume size. Upgrading two disks from 2 to 6 TB will increase the volume size from 6 TB (~5.45 TiB) to 10 TB (9.09 TiB).
General tips here:
When upgrading capacity you want to add disks that are at least as big as the largest drives already in the NAS. So if you upgrade to 2x6 TB + 2x2 TB now, you shouldn't try to upgrade the 2 TB drives to 4 TB later on.
Use NAS-purposed disks (WD Red Plus or Ironwolf) or enterprise class. Avoid the WD Reds. Many desktop class drives in the 2-6 TB range are SMR, which are not good choices for ReadyNAS. Unfortunately, the WD Reds are also SMR (though the Red Plus and Red Pro are good options).
I always fully test new drives before I put them in the NAS in a PC using vendor tools before putting them in the NAS. I have sometimes had drives fail out-of-the-box (likely due to shipment damage)
Netgear recommends making sure you have an up-to-date backup before upgrading. I agree with them (and go further, saying you should always have an up-to-date backup).
