do not share a key between devices. Beside being a security risk, a bad practice, this is the most like reason you had issues. You moved the key but you did not setup the proper file permissions, or did a proper setup of the ssh-agent. If you just generate a new key the gen will take care of that.
Though, if you put your (password-protected) key on something like a (encrypted) thumbdirive you can gain mobility without having to host the key on multiple systems.
If you want to host the key directly on multiple systems, encrypting the filesystem(s) where you're store the key-copies can greatly reduce the cited security-risk.
If you opt to use GPG keys for everything (git-over-SSH, commit-signing, etc.), you can also sub in something like a Yubi-key or other "smart card" type of device. Can probably store and use other types of keys, as well, GPG's just the one I'm most familiar with.
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