IOS 12.4(15)T3 needs over 50MB in flash, so 64 included by default are not enough to support IOS, SMD, and CME.
I wonder if I can replace the Compact Flash using a commercial one (e.a. Kingston). Also because 128 or 256MB are an old fashion item, what could happen if I use a 1GB memory? It is suppoused 256 MB is the maximum flash supported.