You must make sure that the phone is correctly configured to look as an external drive.
Some phones, when not set to act as an external drive, simply don't identify themself correctly, and thus, can't be detected by any host computer.
I've seen at least one case where the phone need to be set to NOT act as a drive in order to function properly: act as a communication device OR act as a drive, NEVER both. That phone had to be switched from one mode to the other every times you wanted to connect it to a computer and transfer anything at all.
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