Macbook os x is playing my external DVD but says it's not there.

Hello,

I am new to Apple/Mac and am experienced with only Windows [Windows 98 thru Windows 10]. My sister gave me her daughter's 2009 Macbook El Capitan with OS X.

I am trying to partition it with Boot Camp to install Windows 7 64 bit Home Premium. I hit a roadblock as soon as I opened Boot Camp. When I hit continue, it read something like "there is no USB device" so I can't go any further. This is all WHILE it is actually playing/reading my Liteon USB external DVD drive perfectly at the same time. It opened up all the files from the Windows 7 installation disc, then I put in two movies which also played great.

So my question is, how can OS X connect with this drive and fully operate it, while saying it's not there?

Using the Macbook's internal DVD drive is not an option as it needs to be replaced [spits out every disc I install, instantly. I cleaned it out with canned air & the cleaning disc ejects the moment it's installed, like all the discs] but that is for another time.

Many thanks for any help you can be.

T'care,

Mike