I have had this problem several times when I put my ssd drive in an install Windows 7 64bit and all the drivers but still in device manager there was an UNKNOWN DEVICE which was driving me up the wall. Today I had some luck. See the Picture below to see is this is what your getting also. If yes click the link below that and download the driver. Install it an no more UNKNOWN DEVICE 

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This is the link below that will fix this problem. I'm so glad I found this today an hope this helps!!!
http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R251237&SystemID=ALW_M17XR2&servicetag=&os=W764&osl=en&deviceid=22270&devlib=0&typecnt=0&vercnt=1&catid=-1&impid=-1&formatcnt=0&libid=41&typeid=-1&dateid=-1&formatid=-1&source=-1&fileid=367209


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This is the link below that will fix this problem. I'm so glad I found this today an hope this helps!!!
http://support.dell.com/support/downloads/download.aspx?c=us&l=en&s=gen&releaseid=R251237&SystemID=ALW_M17XR2&servicetag=&os=W764&osl=en&deviceid=22270&devlib=0&typecnt=0&vercnt=1&catid=-1&impid=-1&formatcnt=0&libid=41&typeid=-1&dateid=-1&formatid=-1&source=-1&fileid=367209





