I have two ideas, the first: What file-system are you using? FAT32 can be formatted to 300 gigs, but NOT USING WINDOWS. I've never tried (or wanted to) use FAT32 on mac.
Second, most drives have size-limiter jumpers. They can force the capacity down for PC's with older BIOSs that can't accept them otherwise. Make sure your drive jumpers are correct.
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I thought of one more thing. If your powerbook G4 is like mine, (alum 1.5ghz or so) the USB really SUCKS. I had endless trouble with my external USB drives on my powerbook. I finally bought a huge firewire disk, and trashed the USB disks. So maybe you can't format on your Mac, because the USB sucks, and you can't format on your Dell, because your trying to use FAT32. That would make a lot of sense.
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I've tried them all. On the Dell I tried NTSF with the USB 2.0. Then I used the DOS command FORMAT and did FAT32. The drive came out about 130GB each way.
Then I tried the Disk Utility that comes with Tiger. Possibly the least useful HD utility I've ever seen. I finally got the drive back to the Apple file system, but still about 129 GB.
I've seen user posts that tell me this drive can go to the 279 GB limit. I had it there once on a PC with NTFS.
I dunno...