How long do external hard drives last, and is mine slowing down a bad sign?

There's no reliable expiration date. A drive can fail early or keep working for many years, and light use doesn't guarantee it'll last. Slowness alone isn't enough to diagnose a failing disk.

Check its S.M.A.R.T. health data with a drive utility, especially reallocated and pending sector counts. Some USB enclosures don't pass that information through, though. A clean health report is reassuring, not a guarantee against failure.

Try a different USB cable and port, then compare transfers using the same large file. A cable, port or the external enclosure can be the bottleneck. Heat during a long copy may also be a factor.

Small files are a different workload from one large file, so comparing the two can make a healthy drive look unusually slow. Fragmentation and where data sits on the disk can affect performance too.

The bigger issue is whether those files exist anywhere else. An external drive holding the only copy is storage, not a backup. Keep another copy on a separate device, and check occasionally that you can actually open the files.

You don't have to pick a fixed decade to migrate everything. Plan to replace drives as they age or show trouble, while keeping multiple copies throughout.

Internal versus external isn't a simple lifespan contest. An external drive may get less use, but it can also get knocked around. Handle it gently and let it spin down fully before moving it. Either kind can fail, so neither replaces a backup.

Fragmentation can slow a mechanical drive, especially with lots of scattered files, so checking it may be worthwhile. I wouldn't make defragmenting the first step on a drive you suspect is failing. Secure another copy of the data first.