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Q:  My G3/G4 Mac has a Western Digital IDE drive and I want to add a 2nd IDE drive as a slave

A:  As noted in my hard drive install guides (Beige G3, B&W G3 rev2 to G4 towers, etc.) at the main site - Western Digital IDE drives (all that I have seen in the last 3 years) have a "Single Drive" setting that must be used if the drive does not have a 2nd (Slave jumper setting) drive on the same cable.
If you install a Western Digital IDE drive with the drive set to "master" and no slave (2nd drive) is on the same cable - it will have boot problems. (For pre-OS X operating systems the mac will usually just show a Flashing "?"/folder icon - for OS X you will get "boot: can't OPEN" and/or "no bootable HFS partition" messages.
Setting the drive to "single drive" will fix this. (often no jumpers on the WD drives is the single drive setting - but check this Western Digital Jumper Settings Page to verify that for your drive model.

If Adding a 2nd Drive on the cable with an existing WD Drive: Unlike Maxtor IDE drives and most other brands, most Western Digital drives require jumper changes to go from a Single Drive setup to having a slave drive (2nd IDE drive on the same cable). If your Mac shipped with a WD drive, you may also need a 2nd jumper for the new setting since many only shipped with one. The following is a reader post from the Feb. 26th, 2000 www.xlr8yourmac.com News page that mentioned the WD drive single vs master jumper settings:

"Mike-
All is well, finally. After much troubleshooting, I finally was able to achieve proper results when testing the Maxtor drive in the G4. There are a couple issues that I think other readers should be aware of when using this drive with G4's that have the WD102AA 10BG harddrive. [It's not just the 10GB WD drive - my G4's WD 20GB Expert drive also required changes for single drive (master) and dual drive (Master with Slave present)-Mike]

The jumper settings on the WD drive need to be changed when installing a second ATA drive. Infact, owners should check to be sure that there are 2 jumpers installed on the factory drive. On mine, there was only one. How foolish of me to assume that OEM drives would come with the proper number of jumpers.... (Some models of IDE drives have a single drive setting that is different from the 'master' setting (sometimes called 'master with slave present' in the past). WD drives are like this usually.-Mike)

There is a page on WD's website that has diagrams of proper jumper settings in various master/slave configurations.
http://support.wdc.com/techinfo/general/jumpers.asp (Link changed by WD several times - this one valid as of June 2004) Doug Shugarts "

For other drive install tips, reviews or guides, see the main www.xlr8yourmac.com storage articles page or the HD/controller topic areas of this FAQ.