Some quick notes in case I ever have to do it again. A WARNING and REMINDER: this took 7 days for a 2TB disk.

One of my backup drives died and wouldn't mount. Really I was using it as an archive as I keep running out of disk space. This is probably a bad idea. Anyway I have rescued it.

My initial thought was using dd but this just kept failing (on unmounted disks; 5 was my new one, 4 the old one, identified using Disk Utility on my mac or diskutil list)

sudo dd bs=4M if=/dev/disk5s2 of=/dev/disk4s1
dd: /dev/disk5s2: Input/output error

A bit of searching round I found a shit article on ddrescue but that led me to the excellent manual page. The first example on there worked for me as a template ("Example 1: Fully automatic rescue of a whole disc with two ext2 partitions").

I used brew install ddrescue then

sudo ddrescue -f -r3 /dev/disk4 /dev/disk5 mapfile

[5 passes for Copying non-tried blocks, 2 for Trimming failed blocks, 3 for Retrying bad sectors]

rescued:    2000 GB,   bad areas:         14,       run time:  7d  1h 14m

then

sudo fsck_hfs -fy -l /dev/disk5s2

** The volume LaCie appears to be OK.
Hooray!