In addition to the Umax story re US driver support, this is what Umax UK say:
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Astra Scanners
Astra 6400 & 6450 scanners.
UMAX are currently Beta testing an OS X VistaScan driver. The Beta version is available for download from the Driver Downloads page.
Currently most Umax scanners driven by VistaScan software will not run in Mac OS X natively, but will run in the Classic environment.
New Astra scanners will soon be released with OS X compatible drivers.
PowerLook scanners
PowerLook scanners driven by MagicScan will soon have a native OS X driver available. It is currently in beta testing due for full release soon. The Beta version is available for download from the Driver Downloads page.
To run your SCSI PowerLook scanner, you must have an OS X compatible SCSI card with an OS X driver/extension. (UK testing has been done using an Adaptec 2906)
3rd Party drivers
We expect independant developers to release OS X compatible drivers to support the older range of scanners over the course of the next few months.
Vuescan - drivers are available for the Astra 3400/50, 5400/50, 6400/50 plus the PowerLook range of scanners.
Silverfast - Lasersoft Imaging have released Mac OS X drivers (version 5.5) for the range of supported Umax scanners. We recommend that users register with Lasersoft and regularly update their driver software.
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I surfed around, and found that Umax products supported by SilverFast include:
http://www.silverfast.com/product/Umax/en.html?target=ai