Cable or OS issue when using iMac 2009 as an external monitor for Macbook Pro 2015?

A 27" Late 2009 iMac running High Sierra can run in Target Display Mode. It needs Mini DisplayPort input.

The Mini DisplayPort to HDMI cable might be designed to connect to a Mini DisplayPort source and a HDMI display. You're trying to use that cable in the reverse of the normal direction.

The current restrictions on TDM say that "the other Mac" has to be running Catalina or earlier.

It looks to me like you may have both a cable problem and a "too new OS" problem. Instead of downgrading your MacBook Pro from Monterey to Catalina (which may prevent you from being able to get current applications for it), you might want to consider getting a real 27" monitor.

Note that with respect to the cable, I think what you would want is a simple Mini DisplayPort to Mini DisplayPort cable. The 2015 MBP's Thunderbolt 2 ports (which can double as Mini DIsplayPorts).

Unfortunately, even with the right cable, you might run into the "MBP's OS is too new" problem.

Hi, starsandtreasures… it’s half OS and half hardware Issues… the iMac isn’t really designed to have a video input and neither is your mid 2015 Mac book pro… plus Target display mode has been discontinued for many years now… and it only supported certain connectors and models of iMacs and MacBook pro’s …here’s apple’s page on it, from 2021 : Use your iMac as a display with target display mode - Apple Support (CA)

otherwise maybe Luna Display/Duet Display is the only recent solution/alternative to that and it’s not free… 😕

You shouldn’t have to downgrade your MacBook Pro ever— if that’s even possible, which it is, but you’d have to have a backup done of your MacBook Pro ( 2015) then wipe everything out, then get a copy of Catalina, install that, then use migration assistant+ Time Capsule to get all your data, apps, And stuff back… seems pretty extreme, huh ???

John B