That’s not possible because of the way disk encryption works. When you unlock an encrypted drive, it does not actually decrypt it - that would take way too long and leave the disk unencrypted. Instead, the computer keeps the key in RAM and uses it to decrypt the accessed data blocks on the fly.

Honestly, I am just so curious about your threat model that you are considering this for self hosting.

The key needs to be available to continue to be able to decrypt the data on the device. All encrypted data is not decrypted as you mount or unlock your encrypted device, that is done one the fly as you use it.

The attack you are thinking of should also not be relevant. What you worry about appears to imply that you are more concerned about the key being protected, rather than the data the key protects. You seem to wish to have your decrypted data available, but not the key.

If an attacker wants your encrypted data that bad, they will attack the running machine and use it to access the data, they will not steal a key and then attempt to physically remove the drive.

Drive encryption is for prevention of access when the drive is offline, it doesn’t protect a running system which can access that data.

If you are worried about the key being accessed while the machine is running, focus on hardening access to the machine via network, etc.

I’m assuming it stays in the RAM till the time the computer shuts downs

Correct.

We know that one could, in theory, get a dump of the contents of the RAM in such a state, if done correctly.

An example of such an attack would a “cold boot attack”.

Is there some way to insert the USB, decrypt the drive, and then remove the USB and all traces of the key from the system?

It sort of depends on how the underlying hardware is designed. You can create a system in which the RAM’s contents are encrypted by the hardware, but at some point the data must be decrypted for use. For example, one could theoretically sniff the data-lines between the RAM, and the CPU. This is all of course ignoring the fact that the hardware, itself, could be compromised i.e. Intel M.E., backdoors/vulnerabilities in the BIOS, etc. There’s lots that can be done to try to mitigate security vulnerabilites, but there is always a tradeoff between security, and convenience.

Maybe the best form of security is memorizing a private key, then manually doing the math with a pen and paper to decrypt some text, and transmit it with a carrier pigeon.

Sharing this article as I think it ties in with this conversation well: https://www.zdnet.com/article/cryogenically-frozen-ram-bypasses-all-disk-encryption-methods/

I do want to say that for most people, this is likely NOT a concern, but I don’t know OPs threat model.