iMac G3 Hard Drive MAX size

That model of iMac will not recognize more than 128GB of a larger hard disk on the internal ATA controller.

(15548)

As Niel mentions, the barrier is basically 120GB as there are no 128GB drives.

That said, there is no limit to an external drive. You can hook up a 750GB monster to your iMac through USB and it will work perfectly fine.

If you had a FireWire port you could even boot from the external drive.

Actually, I have an IBM hard drive in my iMac which has rated capacity of 123.5 GB (a bit unusual). It is model IC35L120AVVA07. I have another Western Digital "120GB" hard drive which, when formatted, is slightly smaller (112 GB versus 115 GB) than the "123.5 GB" IBM drive, so it really is a little bigger.

Also, it is important to note that you can put a 160 GB drive in there and 128 GB will be recognized. It may be wasteful, but someone may have a "larger than 120 GB" drive salvaged from a PC, or there may have been a really good "three coupon deal" at Best Buy on 160 GB drives.