Hi Sandi, there are dozens of different types of cards and machines so could you please provide more information?
You call it USB, but then you call it a card, which is it?
If it is a card, what type is it?
Which machine is it used for?
Do you have software on your computer to see what is on it?
Can you take a photo of it and upload the photo here so we can see how best to advise you?
Would love to help you but don't wish to give you the wrong advice.
hugs and roses, Meganne

It sounds as if you are using a floppy disk, not an USB stick, given the "unknown card type" It might just be the card has become corrupted in one way or another. They are quite old after all. Have you tried other cards? Does that give the same message?
I always copied my expensive bought cards and only used the copies in the machine, so the original would stay in good condition. I hope you made copies or save the contents on your pc.
And sometimes it helps to shut off the machine and start again.

When mine did that i cleaned the steal connection with rubbing alcohol then it worked again***

sometimes it can be the size aka 4GB, 8GB, 16GB, 32GB (if it is a USB) Some machines will not accept the larger GB They just don't have the capacity to open something that size

I get that error a lot, All I do, is remove the card, and put it back in, It normally works the second time round.

What ever type of storage it is, it sounds as if you will lose all the information on it if you proceed with formatting the device for your machine. Please make sure you have back ups of all your files before you try that. It sounds like you were saved from doing that by some sort of write protection on the device. What that protection might be depends on the type of storage device it is. Try the simple advice given below and if those aren't successful, follow Meganne's instructions to get more detailed advice for the specific device you have: a picture is worth a thousand words.
And let us know what happens.