Just so everyone is aware, Roku no longer sells an SD card for the older players that have the slot. They apparently don't feel the need any longer. As has been mentioned already, Roku players with an SD card slot cannot use just any old card. There's no reason to use a card larger than 4 GB, as the Roku will ignore any additional space. And the SD card does not make the Roku perform faster or make channels work better.
The SD card has one sole purpose: to provide additional channel storage space on the player. Some older Roku players have as little as 256 MB of channel storage, while the most recent models all have at least 1 GB (except the streambars have 512 MB). When you install more channels than you have storage space for, the player will offload the least used channel(s) until it has enough space for the new channel. Then if you want to run the offloaded channel, the player has to download and install it again, and it will offload the next least used channel again. So if you have more memory storage, you don't need to download and install the channel again. This makes a faster load for the less used channel(s), but other than that there's no benefit to installing the card. Unless you're constantly cycling through every channel you have installed on your Roku, the occasional delay to reinstall a seldom used channel simply isn't a big deal.
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