The situation.
Disk A. 95% full with Postgres data.
Disk B. 75% full with archival data.
What to do about Disk A?
Move data from disk B to another drive, getting it down to 20% full. Use Postgres’ CREATE TABLESPACE data definition statement to make a new one on Disk B. Run a series of ALTER TABLEs and ALTER INDEXs to safely move half of the Postgres data to Drive B.
Now both drives are at about 60% full and I’m a happy camper with plenty of disk headroom going forward. Bonus, both A and B are SSDs, so with an hour or so of effort, all my data stays fast and my queries might even get a little boost from disk parallelism to boot.