Absolutely this. I do not record to my main Mac drive; I have 2 Glyph external HDDs that I use - one for recording, the other for backing up the recording drive.I've been using Pro Tools for over 20 years, and I know that PT does not work well (often not at all) recording to the same drive as the system is installed on, whether or not it's partitioned. These are symptoms I've seen when not recording to a separate drive. Not sure about other DAWs, but I know for a fact that Pro Tools behaves this way.
Having said that, the new SSD drives seem to have more success in being able to record to the same drive.
This may or may not be the problem, but I thought it worth a mention.