Normally when the Amiga resets, it does a RAM test on the memory at boot-up. There are exceptions though, like when the OS sets a KICKROMTAG (I think) for a particular area of RAM. When that happens, that area of RAM is not subjected to the test on a warm reset. This allows that area of RAM to continue holding the data it held before the reset.
With the RAD disk, this only works for RAM that the Kickstart ROM image knows about, thus leaving out those accellerators that came out after the creation and release of that ROM image.
There is a utility on AmiNET (FastRAD) that can be used to set the appropriate TAGS to allow the RAD disk to work in the accellerator's onboard RAM even if it wouldn't before using FastRAD.
