Disk Too Slow ? System Overload

If it is the internal drive look at the -menu in the upper left corner of the display. Holed down the options key ⌥ while clicking the  and select System report. You will see list of system information. In the Hardware section look for SATA to see the speed of your disk controller. the Link Speed will tell you how many transmissions per second are possible.

If your project is on your internal drive, the problem is not really that your disk is too slow, but that the project is too complicated.

I am seeing "Disk too slow", if project has many complicated software instrument tracks with effects that need a lot of processing. GarageBand may not be able to synthesize the sounds in real time. It helps, if the project is on the internal system drive or on an external drive with a fast connection, directly plugged in, not in HUB or daisy chained. And you may want to quit all other applications, that may be hogging the memory.