Your issue presents a difficult situation. What you place on your employer's computer belongs to your employer. This is especially so if the employer provided you notice of such through your employment agreement or handbook.
Client files are a different thing. If these are clients that were not clients of the firm, but that you represented prior to joining the firm, or handled outside of the firm while employed there, you have a duty to maintain those confidences. You have have breached them by placing them on the employer's computer, but you still have a duty to safeguard them. As to those client files, you should demand their return to you and deletion from their computer. A client authorization would nail it because the firm cannot hold client files upon the client's request.
If on the other hand those client files were for clients you worked on as an employee of the firm, you do not have the right to those files unless the client expressly authorizes the firm to transfer them to you. Before the firm gets that authorization, it is the attorney with responsibility to safeguard those files.
Good luck to you.
This answer should not be construed to create any attorney-client relationship. Such a relationship can be formed only through the mutual execution of an attorney-client agreement. The answer given is based on the extremely limited facts provided and the proper course of action might change significantly with the introduction of other facts. All who read this answer should not rely on the answer to govern their conduct. Please seek the advice of competent counsel after disclosing all facts to that attorney. This answer is intended for California residents only. The answering party is only licensed to practice in the State of California.
With respect to your personal property, consider what the employer's policy was on using their computer system and bringing personal property into the workplace and data storage. Some employers have a detailed and published policy with respect to any personal information being place on their computer systems, a warning about your ongoing access to that information, others do not. If they did not publish a policy to you, I would take the position that the employers failure to return your property is a conversion, much like your having family photographs or personal property you place in your desk drawers confiscated by the employer. If you had a reasonable belief that you would retain access and ownership over those data files, then you might argue you have a right to demand to demadn the return of your perosnal property.
THIS IS A GENERAL ANSWER TO A GENERAL QUESTION AND SHOULD NOT BE RELIED UPON AS A FULL LEGAL ANALYSIS OF ANY FACTUAL MATTER. An attorney-client relationship is not established or offered solely as a result of this answer.