USB to SD interface

Can anyone tell me what might be involved in building an interface device to enable a USB flash drive to be used in an SD slot? I have been searching for information on this but can only find USB devices for reading SD cards. I want to do it the other way around because my DVR has an SD slot but no USB and it's a pain viewing photos through it. Yes, I can insert camera cards without problem but I do a lot of processing on the computer and a direct USB interface would just be so much easier. I'm prepared to build my own but I guess the first question is if this is even possible.

Thanks in advance for any tips.