My friend tried to encrypt his external hard drive with Truecrypt in Windows 7, but he realized that he forgot the password, so he stopped the encryption at ~30%. Is there a way for him to at least recover the other ~70% without a password? A dialog box comes up when the drive is plugged in along the lines of "You need to format this drive before you can use it."
You may be able to retrieve some of the remaining ~70% with data recovery tools but if your friend thinks he stands a chance of remembering the password then another approach would be to complete the encryption process. If he remembers the password then he gets 100% of his data. If he can't, he gets 0%.
According to the TrueCrypt documentation an interrupted system encryption can be resumed. That should (hopefully) also apply to non-system drives providing that the filesystem is NTFS and you're using TrueCrypt 6.1 or later and Windows Vista or Windows 7. I'm inferring this from the FAQ: Can I encrypt a partition/drive without losing the data currently stored on it?
Be aware that any data recovery attempt on the drive prior to completing the encryption may overwrite some of the already encrypted data and render it unmountable/corrupted even if you do manage to complete the encryption process. Best of luck.