Bad Sectors and a Failing (but Working) Drive
A drive that's slow, occasionally throws read errors on specific files, but otherwise still works, is usually failing gradually rather than having already failed — and that's exactly the window where the right response can keep this from ever becoming a real emergency.
What's actually happening
Bad sectors are small areas of a platter, or of flash memory, that can no longer reliably store data — caused by physical wear, media defects that grow over time, or firmware marking a region as suspect after repeated errors. Drives handle a certain number of these automatically, remapping them to spare sectors and logging each event in their SMART data, which is exactly why a drive can be developing real, worsening bad sectors for a long time before anyone notices anything beyond occasional sluggishness.
How recoverable this really is
This one comes down mostly to timing. Files that don't touch the affected sectors are completely unaffected; files that do have unreadable spots in them. More importantly, a drive actively developing bad sectors is one whose overall failure isn't a question of if, only roughly when — so the real question isn't just how recoverable things are right now, but how much time is left before more sectors go, or the drive fails outright.
The general approach
The priority is a full, sector-by-sector image of the drive as soon as growing bad sectors are noticed, using imaging tools built specifically to handle read errors gracefully — retrying selectively, skipping and continuing rather than stalling, reading around damaged areas from alternate directions. Once a good image exists, all further recovery work happens on that copy, and the original drive can be safely retired.
Don't keep the drive in daily use "since it still mostly works," and don't run disk-checking or defragmentation utilities on it. Both add exactly the kind of repeated stress and extra writes that push a slowly failing drive toward a sudden, complete one.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.