disc drive sometimes isn't reconnized by the console
sometimes the disc drive isn't recognized by the console
The Xbox 360 DVD drive is married to the console. You will need to repair the board or reprogram the replacement drive; the method to do it varies based on the firmware of the console, so there's no clear way to do it I can recommend. You need to do this before going on Xbox Live, since a non-matching drive will result in a console ban and treat it as a modified console.
In most cases, the problem is the fuses blow to protect the rest of the drive; I've seen this before. Get a multimeter and see if the fuses by the connector are good. To do this, put it in continuity mode and see if the meter beeps or says SHRT. If it does, the fuses are not blown and the problem lies in another component on the drive PCB. If it doesn't do this, the fuses are blown. DO NOT jump these fuses; this will result in additional drive damage if the fuses blew due to a board or drive PCB failure.
To remove the drive for repair, refer to this guide: Xbox 360 Optical Drive Replacement