Using an old iMac' hard drive as a secondary drive.

mw9850 wrote:

I just upgraded to a 2024 M4 iMac with a 256GB hard drive and would like to access the 1TB hard drive in my old iMac Retina 4K (late 2015). I tried the target disk (holding down the "T" key while turning on the computer) and all that happens is the old computer shows a lighting bolt and nothing happens on the new computer.

That won't work unless you make a Thunderbolt connection. When the Mac that you want to use is an Intel-based one, the following restriction applies.

Transfer files between two Mac computers using target disk mode - Apple Support

"Note: If either of the computers has macOS 11 or later installed, you must connect the two computers using a Thunderbolt cable."

Making a Thunderbolt connection between between the new Mac and the old one will require $80+ of specialized Thunderbolt connection hardware (a $50 Apple TB 3-to-2 adapter and a $30+ Thunderbolt 1/2 cable). (Finding a Thunderbolt 1/2 cable may be difficult these days. A Mini DisplayPort to Mini DisplayPort cable would provide the same physical connection, but might not be high-grade-enough to reliably carry a Thunderbolt signal.)

It's not worth it. You can go out and buy a new 1 TB USB-C (USB 3.1 Gen 2) / NVMe SSD for about $100. The SSD will be much faster than the hard drive in that old iMac, will be more convenient than using that iMac, and will take up far less desk space. (I have a drive like this that is about the size of a credit card.)

On top of that, if that hard drive is ten years old, it may be getting close to the point where it could fail at any time – and take your data with it.

FYI, your M4 iMac does not have a "hard drive". It has a much faster SSD. macOS still uses the name "Macintosh HD" – but that name is outdated and does not reflect the drives that Apple actually ships in new Macs these days.

Is there any way to access the 1TB drive as a secondary or do I have to make it an external drive?

It's your choice, but I would not choose to be penny-wise and pound-foolish in this situation.

You need to be more specific on what you're asking. "I want to access the 1TB hard drive" is vague. For what reason?

Do you want to access the data on the drive to migrate the data from the old Mac to your new Mac?

Are you wanting to continue using the old hard drive?

As others has mentioned, you need to use a thunderbolt connection. Since your old Mac is a Thunderbolt2 system, you need to get the Apple TB2->TB3 adapter so that you can connect the TB2 port from the old Mac to the USBc port on the new Mac. This is going to cost you some $$.

OR, just buy a 1TB external SSD drive for about $100, or preferably a Thunderbolt SSD for a bit more money and have an external drive as fast as your internal SSD.

mw9850 wrote:

I just upgraded to a 2024 M4 iMac with a 256GB hard drive and would like to access the 1TB hard drive in my old iMac Retina 4K (late 2015). I tried the target disk (holding down the "T" key while turning on the computer) and all that happens is the old computer shows a lighting bolt and nothing happens on the new computer. The old computer has Monterey 12.7.6 (will not update to anything newer) and the new computer has Sequoia 15.3.1. Is there any way to access the 1TB drive as a secondary or do I have to make it an external drive?

You could enable File Sharing on the 2015 iMac and access the drive from your new M4 iMac that way. On the M4 iMac you would use the Finder to open & login to the 2015 iMac. (Of course, both Macs would need to be on the same network.)

But even that is not a long-term solution. The drive in your 2015 iMac is 10 years old and should be considered to be reaching its end-of-life. I suggest copying what you need from the 2015 iMac's drive and then get a new external SSD to use with your new M4 iMac.

You might try using Wi-Fi or Ethernet connection via your router.

My M1 will see the hard drive of my 19,2 Imac running Mojave(HFS) and I swap files from and to regularly. You can set up an alias of the 2015 mac hard drive on the desktop of your M4 so it can be opened easily when you want.

You’d need to leave the MacOS on 2015 so it boots but if you don’t want to use it, you can just have one simple account and even delete some applications etc if you want to use more of the 1 TB. Don't forget to take a full backup of the 2015 before/if you are going to delete stuff off it.

mw9850 wrote:

Is there any way to access the 1TB drive as a secondary or do I have to make it an external drive?

you could remove the internal drive and get an external enclosure and use it that way. but the drive in your old iMac is 10 years old. IMHO, I would not bother trying to repurpose such an old drive. if there is data that you want / need on the old iMac, I would move that data to another external storage device. and then you could give the old iMac a well deserved retirement.

DeeperDiver wrote:

OR, just buy a 1TB external SSD drive for about $100, or preferably a Thunderbolt SSD for a bit more money and have an external drive as fast as your internal SSD.

A Thunderbolt 3 SSD would cost considerably more than a USB SSD.

You couldn't hook a bus-powered Thunderbolt 3 SSD directly to the old system (to copy the files off), even with the aid of an Apple Thunderbolt 3-to-2 adapter. That adapter does not transmit power between the sides. You would need that adapter AND a wall-powered Thunderbolt 3 hub or dock AND a Thunderbolt 3 SSD.

The Thunderbolt devices would need to be Thunderbolt 3 ones – not Thunderbolt 4 or 5 ones, (According to an OWC article, Thunderbolt 4 and 5 devices are not backwards-compatible with Thunderbolt 1 and 2 hosts.)

With USB 3.*, edapters to go from USB-A to USB-C (in either direction) are cheap and easy to find online. Thus it would be much easier to attach a USB SSD to the old Mac (to get files onto it) and then transfer it to the new Mac.