Scratchdisk for Photoshop

[wheelhot]

Hi,
I have some questions about scratch disk:

From my understanding, its best to use a second internal hard disk for your scratch disk. Problem is, notebook only has 1 hard disk.

Im currently using PC and hopefully getting a MBP soon. So I will be using Fusion to run Photoshop. I was wondering should I make a third partition for scratch disk or use either an USB2.0 or FireWire(which is better?) external desktop hard disk for my scratch disk?.

Also what happen if I didnt connect my scratch disk when I run Photoshop or Illustrator?

 

[CanadaRAM]

A partition on the same physical drive confers little or no benefit in speed for scratch. The point of having the scratch disk space on a different physical drive is so that the heads of the data drive don't have to interrupt reading/writing data to move to the scratch tracks and read/write scratch, then move back to the data. Since these transactions are highly interleaved, the transit time of the heads becomes significant in slowing down performance. If you have them on 2 separate drives, the 2 sets of heads can stay positioned over their areas of data, and the accesses can be faster.

A partition on one drive still means that the single set of heads has to transit back and forth.

There is a minor advantage to partitioning, and that is it is easier to keep a segregated partition de-fragmented.

If you have the luxury of setting up a drive for scratch use, partitioning it has another benefit -- since scratch disk needs relatively little space, by dedicating a partition for scratch as the first partition created, it guarantees that the scratch files are running on the fastest tracks on the drive (the outermost tracks on the platters). Then you can use the rest of the drive for something (like Time Machine or backup space) which is not as speed critical and won't contend for the heads while you are 'Shopping.

 

[digitalpencil]

Im currently using PC and hopefully getting a MBP soon. So I will be using Fusion to run Photoshop. I was wondering should I make a third partition for scratch disk or use either an USB2.0 or FireWire(which is better?) external desktop hard disk for my scratch disk?.

Also what happen if I didnt connect my scratch disk when I run Photoshop or Illustrator?

Running Fusion for photoshop alone will slow things down a little. I imagine you've bought the PC ver so personally I wouldn't have a problem in running a pirate copy under OS X (it's not like you haven't bought the software title) but that's just me.
As CanadaRAM said, partition on a single disk won't help but it will on a second disk. You can scratch to an external (FW's faster) but it will obviously be slower than using an internal disk. Still works fine though and if you boot photoshop without the disk it'll simply pop a message saying it can't find your scratch disk.
Obviously it depends on the complexity of the images your playing with (no. of layers etc) but I wouldn't worry overly about scratch disks. I use to use one on my old machine but since i upgraded to MBP and maxed the RAM, i haven't had any real issues. FCP needs one but photoshop's been fine.