It still works but am afraid it will unexpectedly die. I bought a new drive that's compatible but don't know how to transfer data from old to new after I install the new drive.
Can I use Time Machine, Disk Utilities or what?
The old drive is Sierra 10.13.4 and the new drive is formatted with Sierra 10.12.
First (after installing the new HD) you'll need to install macOS on the new drive. You can use internet recovery (boot with CMD+OPT+R) or make an install USB.
If you have an external hard drive available you can use Time Machine to create a back up and then restore that backup to your new machine.
If you don't have an external hard drive you could upload your files to iCloud or another online service and restore them later.
Another option is to purchase a SATA to USB adapter (such as this) and either use Migration Assistant or simply copy the files you'd like manually.
On a Mac Pro, this is a no-brainer.
Drop new drive into a spare slot.
Format & partition.
Clone - including recovery partition, which it will ask to add.
Remove old drive.
Continue as though nothing had happened.
[The trick is actually to rename your old drive right before you clone, then name your new drive the same as your old one was.]
Partitions can be any size, so long as the new one is large enough to fit all you data.
