What year is your X5? The older DVD based Nav Drives always take a long time to boot up. Every BMW that I have owned or been in had this issue. My past 2011 and the current 2012 is extreamly fast. I can actually jump in and put the car in reverse and the backup camera actually works.

There is one other thing which I have found that slows the BMW Idrive boot. If you have your BT enabled and sync's with your phone, the idrive will take much longer to boot up. The BMW's BT stack may not have been implemented the best and does cause issues.

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2012 X5 50i ZAP(2VA, 336, 465), ZTP, ZPP, ZPS(6VC), ZCW, ZRC, 322, 4UB, 4NC, 610
2011 ///M3 Silverstone II
2011 X5 50i(lemon)
-- SOLD --
08 550 Msport
07 X5 4.8I sport
04 545i sport
02 4.4i sport
01 540I M/sport
00 SL500
97 SL500