I am going to need to transfer all my projects on the audio HDD in my old computer to the drive in my new computer. What's the best way to do this?

Drive dock or network.

USB HD docking station
Treats HDs like a RAM card...

If its a desktop, just connect the old drive to an empty hard drive slot and power/data cables. You can copy them over, or just leave them on the old drive and redirect Sonar to where they are.

I prefer fireberd's method. Transferring the data is very fast, and even if the old drive eventually gets retired, there are advantages to leaving it in place for awhile. If nothing else, you can use it for project backups.

+1 to both the gentlemen above. I have always left old drives in place until I buy a new machine (at which point I scavenge the best drives from the old one). If on a desktop, you should be able to accommodate 4 drives (including a DVD ROM if you have one).

If replacing an OS drive, an imaging/cloning option would be another step, and not sure if that is simple on Win10 (not done that personally).