How big are the drives? If they're under 1tb you could image them onto (4) bigger (eg 24tb) drives and then scan through those images a lot easier. It's might be worth considering they're lost though if you keep writing to them and haven't put them in areas they'd be protected (or backed them up).

It'd make sense to search for strings like "electrum" and "default-wallet" it'd also make sense to try to work out how electrum stores encrypted and unencrypted wallet files and find a string from those (but I'm not too sure how they're stored).