

Really more of a hardware RAM disk, but CompuPro offered a board called the M-drive for their S-100 ecosystem in the early '80s. 512k of DRAM-based storage; one board cost $1,895 in 02/1983. The potential existed to use up to eight boards in one system, which would give the user a 4MB RAM disk.
FWIW CompuPro targeted the enterprise and scientific markets. Something like this wouldn’t be something a individual home/hobbyist/small business user would be buying.