Talk to your customer, and ask them if you should be keeping copies of their data.
If they don't want you to, problem solved.
If they do, ask them how badly they want to (ie $$$)
If they won't pay the costs, problem solved
If they will, again problem solved
My advice would be to cost out the price of a proper tape library - I know you said you don't want to do that, but honestly if you don't want to do it properly you are taking a huge risk by cutting corners. Tape is one of the best archival forms for storing (you've seen how many people are doing it) and to do it on disk takes more work and effort that you can probably manage. If the customers want multiple copies kept, then charge it at a sensible price and use a commercial storage firm.
It may sound strange, but if you do it properly while you are a small business, then you will have fewer problems when you become a large business. If you are trying to grow your own solution then you are effectively expanding your menu of services to scientific and storage services, or potentially risking becoming a small business that failed.