Only 1 of my disk drives work at a time

The title says it all i just decided to put in a old disk drive i have when i say old i got it last year but then i got another new one :blink: . The problem is when the old one is installed the one that was already installed does not show up in either device manager or my computer. The power works i.e. i can eject the tray etc. but nothing else i then with out touching the already installed drive unplug my old drive and the one that was already installed works.

Anybody got a clue what's going on, thanks.

Could they both be set to primary and it is just readig witch ever one is put in the primary plug on the cord connecting it to your computer?

maybe, the PSU.