Lost access to reference originals

I upgraded OS and Aperture to Yosemite after sending the originals (using reference command) to a WD external drive. It turns out the WD drive is not compatible with Yosemite and now I can't access the originals.

It is probably the WD software on the drive, not the drive itself. You could copy the files temporarily to another drive, that both Yosemite and your PC can access, using your PC. After you have saved the content to another drive, reformat your WD drive to MacOS Extended (Journaled) and copy the originals back. This will only work, if you do not need the WD software, and do not need to use the WD drive with Windows systems. Reformatting will erase the drive.

See this link: Format external drives to Mac OS Extended before using with Aperture

After the drive has been formatted correctly, and the files ave been put back into exactly the same folders a before, try to reconnect the originals. If the drive has the same name as before and the files can be found along the same paths, "Reconnect All" should work, so you do not need to reconnect the originals individually.

See this manual page: http://documentation.apple.com/en/aperture/usermanual/index.html#chapter=5%26sec tion=16%26hash=apple_ref:doc:uid:Aperture-UserManual-91292BSR-1037600