What is the best (faster yet efficient) Anti-Virus scanner and virus removal tool for external hard drives?

As far as anti-viruses/virus removals/etc go:

• For Windows & macOS, I would just enable and use the built-in anti-virus protection (Defender on Windows, XProtect on macOS).
• On macOS, I would also enable and use the built-in firewall (Nearly every other OS already comes with a firewall enabled out of the box, no idea why macOS doesn't enable theirs by default...), as well as Lockdown Mode if you're able to.
• On Linux, I would use ClamAV & ClamTK.
• On Android, I would use Hypatia & Auditor.

I would also strongly recommend making use of DNS level protection through a service like NextDNS, ensuring you have a good content blocker like uBlock Origin in your browser, and using Safe Browsing in your browser (As long as you use a good browser like Brave or Firefox, then Safe Browsing won't endanger your privacy, it just improves your security and protection, so I'd recommend using it).

As far as encryption goes, I would recommend just using whatever is built-in to your OS, such as BitLocker on Windows, FileVault on macOS, & LUKS on Linux. You can also use VeraCrypt if you wish to as well, may be preferable in some cases, though I personally don't bother.

Assuming you’re on Windows and want something other than Defender, you can try Malwarebytes. For encryption, you can use Veracrypt.

I only know an answer to this:

Can you also suggest a good opensource/affordable encryption software for hard drives?

Veracrypt is the most used

For personal use? Just rely on Windows Defender and common sense to avoid getting viruses. If you already have (or suspect) a virus, there's many guides out there for virus removal and most parts will apply to external hard drives.