iMac early 2009 24 inch upgrade hard drive

Assuming you can find the room inside the computer it would work, however I'd put the Time Machine backup drive in an external enclosure and use that as Time Machine is designed to work on External Hard Disks.

You don't mention anything about how you intend to install Mac OS so I'm assuming you have the original install discs or a Snow Leopard upgrade DVD or a bootable clone you can use. Without these you're pretty much dead in the water as your machine is too old to work with Internet Recovery!

I would also refer to www.macsales.com as your source for parts and install information. They specialize in Macs and can show you how to take it apart (it's doable but not simple) and can provide the correct tools, adapters, enclosures etc....

Also remember that the furthest you can take this machine is 10.11.x as it simply does not meet the hardware requirements for more current versions of Mac OS.

Just want you to go into this with your eyes wide open and aware of what you are getting into.

Just an FYI, I'm running OS X El Capitan version 10.11.6 on an iMac (24-inch, Early 2009) using a 1 TB Samsung SSD as the Startup Disk in an external Oyen Digital Mini Pro RAID V2 2.5 inch drive enclosure -- externally via the FireWire 800 port -- and despite being an external SSD boot drive the speed / performance increase is very significant over the internal HDD. The boot up time is significantly faster than the internal HDD, and once booted the system overall is even more dramatically faster in speed / performance than the internal HDD. Easy and quick upgrade without even needing to open up the iMac case. I cloned the internal system HDD onto the external FireWire 800 SSD (solid state drive) and it has been working great for about 6 months so far. Note: I also tested booting from an external system SDD via a USB 2.0 port and it also worked okay -- but of course much slower than running via the FireWire 800 port. At some point I may get around to actually opening up the iMac and installing the Samsung SDD system drive internally in either the HHD or DVD bay to take better advantage of maximum potential speed / performance via the internal connection -- but no rush as booting / running the system and software via the SSD off the external FireWire 800 port was so quick and easy and is still such a very significant and dramatic speed / performance boost.

Just an FYI, I'm running OS X El Capitan version 10.11.6 on an iMac (24-inch, Early 2009) using a 1 TB Samsung SSD as the Startup Disk in an external Oyen Digital Mini Pro RAID V2 2.5 inch drive enclosure -- externally via the FireWire 800 port -- and despite being an external SSD boot drive the speed / performance increase is very significant over the internal HDD. The boot up time is significantly faster than the internal HDD, and once booted the system overall is even more dramatically faster in speed / performance than the internal HDD. Easy and quick upgrade without even needing to open up the iMac case. I cloned the internal system HDD onto the external FireWire 800 SSD (solid state drive) and it has been working great for about 6 months so far. Note: I also tested booting from an external system SSD via a USB 2.0 port and it also worked okay -- but of course much slower than running via the FireWire 800 port. At some point I may get around to actually opening up the iMac and installing the Samsung SSD system drive internally in either the HDD or DVD bay to take better advantage of maximum potential speed / performance via the internal connection -- but no rush as booting / running the system and software via the SSD off the external FireWire 800 port was such a quick and easy upgrade and is still such a very significant and dramatic speed / performance boost. Could not be happier.

Something of note is that the early 2009 iMac has a SATA II

interface. You did not mention the make or model of SSD,

but most are usually SATA III interfaces.

SATA III is backward compatible to SATA II, but there is no

a requirement that drive manufacturers adhere to this. In

other words, verify wit the manufacturer that yours will

work on a SATA II bus.