How to move old data to cheap hard drive
Here is a better architecture:


Create a new tablespace on the slow drive.
Set the storage parameters seq_page_cost and random_page_cost higher on that new tablespace so that the PostgreSQL optimizer knows that the disks are slower.
Partition the big time series tables by time ranges (use the same boundaries for all affected tables) so that you end up with a couple of dozen partitions for each.
Move the old partitions to the slow tablespace.


Then you still have all the data accessible.

Use PostgreSQL v11 or better for partitioning.