Remove old HDD from iMac and mount on another Mac

Thank you. The safest way with your combo of Macs is to use a bare drive adapter similar to this:

https://eshop.macsales.com/item/NewerTech/U3NV2SPATA/

It lets you mount a pulled working drive on another computer just as if it were an external hard drive. If a 2.5-inch drive, the adapter supplies the power; if a 3.5-inch drive, the above kit comes with an auxiliary power connector and supply.

Although I use the above model, there may be less expensive versions on sites like Amazon. Just make sure the if yo have a 3.5-inch drive, the kit come with the external power supply.

To connect to ports on your 2021 Macbook Pro, you will also need a USB-C to USB adapter:

USB-C to USB Adapter - Apple

Our knowing model YEARS is critical to resolving this issue remotely. iMacs have been in continuous production since 1998 in five broad families with differing specs. “Old iMac” is overly broad.

— From what model year iMac did the old drive come?

— What OS version was the old iMac running?

— What current computer models/ years do you have available to be the recovery vehicle?