If it were me, I would give satnav_tecknik's approach a quick try. No harm there. But, if that does not quickly produce results, then I would go with BMW's official method that BabyMakingWeather quoted from BMW's Phone and Navigation Systems Technical Service Information Bulletin. (Incidentially, BMW has been publishing this same process for recovering from a failed OS installation since at least early 2005. There may be other methods, but I trust this one too.)

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input