D Drive should be straight copy of data.. and hopefully can pick up anything else on the C
Ah, didn't realise you had good backups. The reference to non-booting old system made me think the old HDD was the main/only copy of important stuff (very silly presumption as you're obviously so professional in other ways
)
I "prefer" to work directly off the old HDD IF it's in good condition, otherwise from backups (or recovered data copied to other media from a dodgy drive).
Whatever the source, assuming the new system has a larger faster disk, I bulk copy "almost everything" first to a folder on the new system then work from that.
Two main reasons for that:
1. minimise "touching" of your precious original/backup in case you need it again.
2. It's usually faster to rummage/reorganise totally on the new faster hardware.
Hope it all goes to plan.