The largest HDD that is available at the moment is the 20 terabyte Seagate drive. This is a massive amount of storage. NTFS can support volumes as large as 8 petabytes on Windows Server 2019 and newer and Windows 10, version 1709 and newer (older versions support up to 256 TB). So this could be formatted to the maximum capacity on a modern Windows system. An EXT4 file system supports a 1 Exabyte partition with a 4 KiB block size. Still quite impressive. Storing so much data on one HDD is prone to failure, so maybe include some mirroring of data to minimize losses in case of a failure. Still, this is an impressive amount of storage space for all of your data. very useful to have in a NAS for backup purposes. A 1 Exabyte partition would not fill up for a very long time. Will we ever get better HDD technology than spinning hard disks and SSD hardware? This would be very interesting. There has to be a more reliable way to store data and have it retrievable in the future.
The largest SSD currently available is the 8TB OWC Accelsior 4M2. This is a Full Height Half Length PCIe card that is supported on Linux using kernel 5.4 or later. Impressive. And it runs at 6,000 MB per second. Very useful speed for intensive tasks.
There is a 200 Terabyte SSD in development, but this may be a while away yet. Time will tell when this is actually available, but it would be very expensive.