AFAIK, your only real option is to get SFF drives. Sometimes, the smaller and faster LFF drives are actually SFF drives in a larger carrier. If this is the case then you can by SFF carriers and swap them. If not, you really should be looking at purchasing drives compatible with the system. Sometimes you can swap/add drive bays but I don't think that's an option for you and it's probably going to cost you as much time and frustration as just getting compatible drives.
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