Is flight data recorder erased after every flight?
Flight data recorders have changed over the years and evolved to what they are today. Modern recorders are recording up to 88 parameters. The storage of the recorder is dependent on the range of the aircraft. Early recorders were using a circular tape that ran through the recorder. E.g. loop recording all the data and automatically overwriting the oldest data on the tape.
The recorder of a Boeing 777 is based around modern solid state storage, capable of recording up to 25 hours continuously. Modern flight recorders are still recording continuously, so after the initial 25 hours, the storage is full. It will then start overwriting the oldest datablock stored. This ensures that you will always have the last 25 hours of data before a crash available for research.
The following quote comes from AERO Magazine Issue 02 - Spring 1998:

Flight data recorders were first introduced in the 1950s. Many first-generation FDRs used metal foil as the recording medium, with each single strip of foil capable of recording 200 to 400 hr of data. This metal foil was housed in a crash- survivable box installed in the aft end of an airplane. Beginning in 1965, FDRs (commonly known as &quot;black boxes&quot;) were required to be painted bright orange or bright yellow, making them easier to locate at a crash site.
Second-generation FDRs were introduced in the 1970s as the requirement to record more data increased, but they were unable to process the larger amounts of incoming sensor data. The solution was development of the flight data acquisition unit (FDAU).
As shown in figure 2, the FDAU processes sensor data, then digitizes and formats it for transmission to the FDR. The second-generation digital FDR (DFDR) uses tape similar to audio recording tape. The tape is 300 to 500 ft long and can record up to 25 hr of data. It is stored in a cassette device mounted in a crash-protected enclosure.
FAA rule changes in the late 1980s required the first-generation FDRs to be replaced with digital recorders. Many of the older FDRs were replaced with second-generation magnetic tape recorders that can process incoming data without an FDAU. Most of these DFDRs can process up to 18 input parameters (signals). This requirement was based upon an airplane with four engines and a requirement to record 11 operational parameters for up to 25 hours (see &quot;Parameters Explained&quot; below).
Another FAA rule change that took effect October 11, 1991, led to the installation of digital FDAUs (DFDAUs) and DFDRs with solid-state memory on all Boeing airplanes before delivery. This FDR system was required to record a minimum of 34 parameter groups. The DFDAU processes approximately 100 different sensor signals per second for transmission to the DFDR, which uses electronics to accommodate data for a 25-hr period.
Today all Boeing current-production models use DFDR systems, which will store 64 12-bit words per second (wps) over a 25-hr period in electronic memory. At the end of the 25 hours, the DFDR will begin recording the most recent data over the oldest data. No tape removal is required with these systems. Each of these systems on every Boeing model (except the 777) have at least two data frames that are transmitted from the DFDAU to the DFDR (see &quot;What Is a Data Frame?&quot; below).
These separate data frames accommodate the different regulatory agency requirements. A 128-wps DFDR was available for the Boeing 777 and MD-90, allowing the development of one data frame that incorporated all regulatory agency requirements and that required operators to develop only one data frame decode algorithm. &quot;How a FAA Rule Is Changed&quot;, below, explains the basis on which the FAA may propose rule changes.

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