I've had no direct experience with the SD-based MMC systems, however I think that they're like the earlier DVD-based ones which don't have full recovery software on the map discs so if the system is powered down for more than a very short time the head unit loses it's GPS firmware [unit software] held in volatile memory. Only an OEM disc with full software contained in LOADING.KWI can then write back to volatile memory, an update 'map data' disc can't. If that's correct also for the SD units [it's only my supposition] then provided you can find the correct KWI file you can write back easily.
Of course, holding the firmware in volatile memory on the face of it seems illogical but in fact it's a fail-safe because if the fw becomes corrupt, or if the wrong fw is used, then it's very simple to remove it by disconnecting power. When the unit reboots it then asks for the correct disc.
BTW, we discourage using the forum for selling or barter. Actually it's prohibited. See General Rule "9.o" of [Only registered and activated users can see links. ]. This forum is based on the pure principal of 'help and be helped freely'. Free in both senses, without restraint and without expectation of profit or reward [other than the contributor being helped in turn by others if needed]. If someone has a repair disc, we would ask that they make a copy and upload it. I'm aware other forums allow some selling but here it's just not on at all. I hope you understand.