Windows – I’ve lost access to the encrypted files

The EFS keys are kept in NTFS alternate data streams, so if your USB drive was not formatted as an NTFS drive, the information would be lost. Backup programs often discard this information as well.

You can check if the streams are still there using Streams or ADS Spy. If they are no longer there, it will be impossible for you to recover your files.

As Windows does not flag the files as EFS encrypted, I think it is likely that you have lost the EFS keys in the streams.

The existing answer is correct in that the EFS private key is protected by the user's password. However, it is possible to configure EFS Data Recovery Agents that can decrypt any EFS-encrypted file on a system. DRA certificates are set via Group Policy, or Local Security Policy if you don't have a domain.

DRAs have such access because when a system receives the public key of the DRAs, it encrypts the symmetric key of each encrypted file with each DRA's public key in addition to the user's public key. Thus, DRAs can only recover encrypted files if they were created or opened after their certificate was registered.

So, depending on your configuration, it could be possible to recover the data even after resetting the owner's password. DRA keys are also protected by the DRA's password, but a crafty attacker would install a DRA certificate for a new user, wait for you to touch the target files, then take advantage of the certificate to decrypt them.

Note that this recovery option does not apply to DPAPI-protected data, as the DPAPI does not respect EFS DRAs. You're in for some pain if you need to recover such data.