There are some streamers that do allow you to plug a USB drive in for external storage; the Holo Red is not one of them, which is unfortunate since you already own it. The iFi Zen Stream is one of them. (I ordered one and the box is sitting here since it came earlier today— I’m not yet able to give any details about how well it works. But it has only USB out, which is not ideal for the Mk 1.) There is a lengthy thread here about which streamer to use with the DS DAC. You can find a lot of useful information there even though you were still using a Mk1.