The job of "consolidating" from multiple old hard drives that very likely contain much duplication is a job best left to YOU to fully control, evaluating information presented to you by very capable software tools and utilities.
As was already expressed, Free Commander has some good capabilities in the area of comparison/sync.
But for truly superb such folder/file capability, along with the sought-after "sync" and "mirror" (from left to right, from right to left, both sides, etc.) functions that provide ultimate flexibility (under your direction but performed flawlessly by the program) I highly recommend Beyond Compare
This program supports not only folder and file (i.e. directory) comparisons, but also can compare on CONTENT (text and binary), showing you the differences highlighted with colors, and allowing you to selectively copy portions (i.e. lines or fields, etc.) of that content from one-side to the other, etc.
The user-interface of Beyond Compare is very easy to understand and make use of.
So for your purposes, it might be just the ticket... finding not only absolute duplicates that might be on multiple drives (perhaps in folders that have different names) but also allowing you to conveniently examine file CONTENTS from what you suspect might be duplicates (or maybe even near-duplicates), so that you can decide for yourself WHICH one you'd like to end up with and which one you can delete in the consolidation process.
Highly recommended. But YOU are in charge of this, and the program is simply your tool.