External Drive maximum size for 2011 iMac

I doubt you have a VERY expensive 20TB SSD connected to a $200 used iMac. Plus, if you do have that much capacity and assuming you intend to use a large number of files big and small, the low speed USB2 connection would make it close to unusable.

How do you know every file is corrupted?

It's a firmware trick to report a larger capacity than it has. It's unfortunate but there has been a flood of fake high capacity SSDs on the market for the last few years. Amazon still sells them. Walmart at least had the decency to pull the ads.

Are you certain you have a 20TB SSD? They would cost $3500 and up which is quite a lot to pay for an obsolete iMac.

Is this it???

Novachips 20TB SCALAR-20T 2.5-inch SSD unveiled - Geeky Gadgets (geeky-gadgets.com)

That is NOT a 20TB SSD drive, I can assure you. I suspect you were the victim of a scam.

It's a 64GB microSD card in sheep's clothing...

Inside a fake $20 '16TB external M.2 SSD' | ZDNET

You're welcome. I did manage to find a couple 16TB external SSDs for $2100-$2500. 😎

Yep, there've been plenty of fake USB Flash drives advertised at huge sizes for years that turn out to be just a few MB in reality, but simply flashed with a big number.:(

Some are clever enough to write over themselves over and over. That way it looks like you can write multi-TBs to them and only end up with 64GBs or less. Bad them.