In the last month or two the SD card on my i760 randomly disappears. The card works "most" of the time (running WM 6.1). After scouring the net for a couple hours, I finally tracked down this:
Fix SD Memory Card Disappearing, Missing Folder or Corrupt in Windows Mobile Device » My Digital Life

Just change a couple registry entries:
[HKLM\System\StorageManager]
“PNPUnloadDelay“=dword:8196

[HKLM\System\StorageManager]
“PNPWaitIODelay“=dword:8196

Seems to work fine now!!!

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