Let's say you have 3 drives X Y Z
X has 100 GB of free space you would be ok using for the blockchain
Y has 300 GB
Z has 100 GB
Would it be possible to tell the client, "allocate 100GB to X, 300GB to Y, 100GB to Z" and have the files downloaded and verified in the right order?
And you could just keep adding further drives in case you ran out of space. Right now if you run out of space you have to buy a bigger device that allows at least 500GB at least once in case you used pruned mode, but for someone that wants to keep the entire blockchain this would be good. It would also give some usage for smaller drives you don't use much.
As said, drives are cheap. So there's no reasonably good reason for implement anything like you said.
For now there are settings in config for having different folders in different places or you can use symlinks. However, this means that you'll still need a drive for storing the blocks folder, which is ~477 GB and another drive(s) for the rest.
Right now getting a HDD for the bulk of data (I've bought not long ago a 2TB a CMR HDD exactly for this) is not expensive and, if you set up the chainstate (and indexes) stay on SSD (at least temporarily) the IBD is also relatively fast. So imho doing odd setups with the HDDs for the sake of free space for bitcoin doesn't worth it.