I’d suspect that too. Try just reading from the source drive or just writing to the destination drive and see which causes the problems. Could also be a corrupt filesystem; probably not a bad idea to try to fsck it.

IME, on a failing disk, you can get I/O blocking as the system retries, but it usually won’t freeze the system unless your swap partition/file is on that drive. Then, as soon as the kernel goes to pull something from swap on the failing drive, everything blocks. Might try swapoff -a before doing the rsync to disable swap.

At first I was under suspicion was temperature.

I’ve never had it happen, but it is possible for heat to cause issues for hard drives. I don’t know if the firmware will slow down operation to keep temperature sane — rotational drives do normally have temperature sensors, so I’d think that it would. Could try aiming a fan at the things. I doubt that that’s it, though.