I have a G3 400mhz 612ram and am running out of space on the HDD and am relativly new to Mac. Can I add another HDD or should I get one with a larger capacity?
Yes, you can add a HD to the G3. It is fairly easy. However, I agree with bauman, adding an external FW drive is far easier.
One idea to consider is to get a new HD, use it to replace the existing HD and put the old one in an external FW enclosure.
HDs have gotten a lot faster and Apple usually doesn't use the fastest drives. You will see a healthy improvement in performance by swapping drives.
One disadvantage of having two drives in one system is that the second drive will spin down due to disuse. Then when you select some finder function it will try to check all drives before proceeding resulting in a spinning beach ball for a few seconds from time to time during the day. With an external FW drive you can turn off the drive and avoid this problem.
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One idea to consider is to get a new HD, use it to replace the existing HD and put the old one in an external FW enclosure.
HDs have gotten a lot faster and Apple usually doesn't use the fastest drives. You will see a healthy improvement in performance by swapping drives.
One disadvantage of having two drives in one system is that the second drive will spin down due to disuse. Then when you select some finder function it will try to check all drives before proceeding resulting in a spinning beach ball for a few seconds from time to time during the day. With an external FW drive you can turn off the drive and avoid this problem.