I am using a DS920 and have 4TB in all 4 bays, almost maxed in storage, i want to upgrade my HDD but unsure how. do i just take the HDD out and put in a new one?
Also how big can i go i see now HDD can go up to 20TB, just curious if i want to future proof can i swap 1 to 20TB and slowly replace each bay as space runs out?
The key information missing in your post is the storage pool RAID type.
Redundant RAID types, like SHR and RAID5, let you swap an HDD one at a time and let it rebuild. Non-redundant types like RAID0 and JBOD, won’t let you do that.
With SHR, you will see the storage pool extend from two larger HDDs, and with RAID5, after replacing all HDDs.
You find the storage pool information in Storage Manager.
I am unsure what the maximum pool size is for the DS920+, but I assume you looked that up in the specs. Since the DS920+ was introduced, larger capacity disks that are not on the compatibility list have become available. They might, however, work well.
I have a 2 bay DS218+ w/6GB RAM and it is running JABODa with the 2 being a single volume to back up my big drive. It is running 20TB drives with no problems. (if your question was can my Synology can handle 20TB drives).