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Yes, you can use 48X CD-R on a 24X drive. Numbers printed on CD-Rs means maximum speed allowed, when you use 48X CD-R on 24 drive, you will only operate at max speed of 24X.
 
Some cd-r's say 48x 52x and so forth. That number only means the maximum speed those cds will burn at. Your cds will burn under your 24x burner.

Say I have a 52x cd burner and I have 48x cd-r. I cant burn at 48x cds at 52x speed.
 
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