We recommend that you use a USB 3.0/SSD drive rather than an SD card for multiple streams or for bit rates greater than 10 Mbps. SlingStudio currently supports a maximum of six streams (four input streams plus a program recording and a quad view recording) and bit rates of 1.5 to 30 Mbps.
If your project involves very high bit rates and you cannot risk having frame drops (which can lead to AV sync issues), an enterprise SSD drive is recommended for storage instead of an SD card or standard USB 3.0 drives. See the article linked below for recommended USB/SSD drives.
Just as a general note: It is not necessary to have an SD card inserted into your SlingStudio unit (or a hard disk attached) to stream live video. You can stream uninterrupted even when storage media is removed, becomes full, or experiences an error.
You only need storage media present if you're working with a project that requires actual storage.