Note that all of this is in the context of backups; duplicates for the purpose of restoring the originals in case something happens to them. Though it is at least possible to use an index cold storage system like what I describe for more frequent access, I would find that very inconvenient for “hot” data.

how would you use an index’d storage base if the drives weren’t connected

You take a look at your index where the data you need is located, connect to that singular location (i.e. plug in a drive) and then copy it into the place it went missing from.

The difference is that, with an Index, you gain granularity. If you only need file A, you don’t need to connect all 12 backup drives, just the one that has file A on it.