If your external hard drive is fully disk encrypted, is it normal that when you connect your drive to the computer, it doesn’t show up where you normally find all of your connected devices, that i’ll only show up once you mount it and decrypt it in VeraCrypt? Is that a good thing?
You need to define what you mean by “decrypt”, if you mean that you need to somehow tell when mounting what passphrase/secret key you used to the OS and all the operations then with that disk will encrypt/decrypt data on the fly, sure, this is why you bother with it.
If you mean that you have to wait overnight (or even days) for the disk to get decrypted, and then it’ll be all clear text, no, that shouldn’t be happening.