Can I add an old hard drive to my ThinkCentre for more storage?

First check which M73 case you have. The model came in different sizes, and the tiny version may not have room for a second internal drive. The manual for your specific version should show the drive bays and connections.

If there's room and the old HDD is a SATA drive, you need a mounting spot, a SATA data connection to the motherboard and a SATA power connection. A SATA data cable may not be included, and there needs to be a spare power connector or a suitable way to provide one. Shut down and unplug the PC before opening it.

Keeping the SSD as the boot drive and adding the HDD as secondary storage is usually the simpler setup. Once connected, check Windows Disk Management to see whether the drive appears. Don't format it if it contains anything you want to keep.

If the case doesn't have space, a USB enclosure or SATA-to-USB adapter is an easy alternative. Match it to the drive's size and connection. A 3.5 inch desktop HDD normally needs an adapter or enclosure with its own power supply.

Before moving the disk, check whether there's anything on it you want to save. If it's encrypted with BitLocker, make sure you have the recovery key, or decrypt it before moving it. Otherwise Windows may ask for the key when you connect it to the other PC.

Another option is replacing the small SSD with a larger one instead of adding an old HDD. That costs more, but gives you more room on the main drive. You'd need to transfer or reinstall Windows if you replace the boot drive, so it isn't just a matter of swapping disks.