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RAM levels on storage varies quite a bit, but a brief search suggests the following pattern:
512 MB of RAM = 14 TB HDDs
256 MB = 2 TB to 16 TB
128 MB = 500 GB to 8 TB
64 MB = 250 GB to 6 TB
32 MB = 160 GB to 4 TB
There's even more RAM on SSDs, though:
4 GB of RAM = 4 TB SSDs
2 GB = 2 TB
1 GB = 512 GB to 1 TB
512 MB = 128 GB to 2 TB
256 MB = 120 GB to 960 GB
128 MB = 64 GB to 1 TB
64 MB = 60 GB to 240 GB
32 MB = 60 GB to 120 GB
So the rate of expansion is pretty slow on HDDs, but it's ballooned on SSDs - this is because NAND flash aren't very quick, takes place in blocks (so small data writes have to be buffered until a block is full), and the memory cells wear away with every write. So lots of DRAM helps here, as data can be preloaded, small data packets held ready to put on, or just kept to one side to reduce the wear.