From RHESSI Wiki
The SSR (Solid State Recorder) is RHESSI's on-board hard drive. It stores photon events detected by the Germanium detectors with a maximum capacity of 4 Gb (or 10^9 photons events). Data are written and read out through two pointers, the read pointer and write pointer. The normal mode of operation corresponds to the read pointer chasing the write pointer (Note that the read pointer only moves during downlinks while the write pointer records continuously). The SSR is a cyclic FIFO (First In First Out) device, meaning that earliest data will be read out first and that the end of SSR is also its beginning.
In principle the read or write pointers for the SSR can be moved, and this is sometimes done in order to delete data from telemetry if downlink capacity does not keep up and if activity is high.