I notice in CS4, that in the Performance Preference, Photoshop is giving me an option to use my 2 external drives as scratch disks. Is it now okay to do this? If not, what happens if you do?
PS always has allowed that. Primary scratch should be a fast drive physically different than the OS/apps drive, ideally a fast internal drive. If an external use the fastest drive and connectivity you have available: eSATA best, FW800 second choice, FW400 third choice; avoid USB.
It has always been OK its just that an external drive is not as fast as an internal drive but if you only have one internal (system drive) the you need an external scratch.
As a sidebar, some folks may not realize that USB2 — despite its specs to the contrary — is slower than FW400 due to its greater overhead. And generally speaking, if you wish to have an external drive that is bootable (a useful option should your internal startup drive bite the Big One), you need FW400. In the long run, any saving from purchasing a USB2-only drive is not worth it.
If you hadn’t seen this option before, it means only that you probably had attached external drives that were not formatted to Mac HFS+ (Extended, Journaled). A FAT32 drive, for instance, does not work.
Is it now okay to do this? If not, what happens if you do?
The problem that you may run into if you select External Drives for Scratch would arise if you later disconnect those Drives and Photoshop posts a "Disk not Found" error message.
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