I want to upgrade my late 2009 Imac internal drive to 4 TB at 7200 RPM.

Some external disks don't use normal SATA connections. So you may not be able install it into your iMAC. On the other hand, if the external disk comes with firewire, you could use that directly with your iMAC without the need for cannibalization.

Usually an external drive the vendor includes whatever drive they want. It might be due to overproduction, .... Or old/er drives that did not sell...

IMHO, it might be more beneficial to get an external, NAS that includes multiple drives and supports RAID capability.

Backups are always ..

Look at replacing the full 3.5 inch HD with two 2.5" drives 256Gb SSD and 2Tb HD.

Are you saying that I could fit 2 2.5 drives in one 3.5 space?  That's 5 inches in a 3.5 inch space. Also, where would the extra sata connection and power connection come from? Or are you suggesting replacing the optical drive for one of the shorter drives?

Apple doesn't want you upgrading the computer - they want you upgrading to a NEW computer.  If the internal board doesn't support a second SATA connection, then there's no point in using 2x2.5 drives.  And don't add them together - the 2.5" drives are as little as 7mm high... you can easily stack two on top of each other with a $5 mounting bracket.  

But you may be facing another problem.  The firmware of the mac may not correctly recognize drives larger than 2TB.   On a PC, your PC must have UEFI enabled to boot to a disk larger than 2TB or to make the entire disk available.  Even if it appears to work, you may not be able to use any space above 2TB.

That with booting to GPT disks requiring an UEFI BIOS is only true for PC with Windows. There is no problem if you happen to be running Linux, then you can also boot from GPT disks (which can be larger than 2TB) using a conventional BIOS.

On the other hand, iMAC's from 2009 will all have an EFI firmware that supports 64 bit OS's. The last ones that did only used a 32bit EFI firmware used 32 bit Core DUO CPU's (not Core 2 DUO's), and those were from around 2006. So disk sizes and UEFI should not be an issue with an iMAC from 2009.

Wow, I totally forgot about that particular aspect as a possible problem. And sorry that I didn't know that 2.5 inches was just a convention, not reality.

I'm having trouble distinguishing between all the various flavors of internal drives, particularly the WD drives. My major use for the IMac will be as a DAW. Any advice as to which kind of drive (surveillance, NAS, high RPM, etc.) would be best for this application?  Or should I post this as a new question? As a complete Newby, I don't want to unwittingly violate the etiquette of the forum.  Any advice is greatly appreciated! And thanks for your kind attention.

A 3.5 drive is 3.5x1inch high, the 2.5 have a night 7mm slightly less.there are no moving parts so
The length of the 3.5 is around 5 inch
Yes giving up the optical ...

Is some of the message missing?

Not on my part is just a full evaluation of what it is you want to do? Getting a NAS device might be simpler and achieve what you want....

I just gave up and bought a 4 TB Toshiba internal drive.