External HD not appearing in Preferences/ScratchDisk Dialogue

I want to use an external (firewire) hard drive on my iBook with OS-X and PS CS but the hard drive does not appear in the drop down list in the Preferences' scratch disk list. The iBook does recognize the external hard drive in the File Windows and I can access images on the external hard drive from PS CS, so I know that the system is functioning. The external hard drive shows up in the File Window as "ejectable" ie...the triangular ejection arrow shows up next to the hard drive icon in the File Window and I can press on the ejection arrow which will make the hard drive icon disappear as though it were a CD in the CD Drive. This is a mystery to me, anyone have a solution for this?

If you haven't already doesn't so you will need to initialize the disk as Mac HFS+. They're normally supplied formated for the PC and whilst the Mac can read the disk PS treats it as a removable drive - thus not an acceptable option as a scratch disk.

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There are known unknowns.
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We do not know.
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