Jan 3, 2019 - Here are the best Mac hard drives and external storage options for your Mac. Unlike some of the bigger models in the range, the Rugged Mini. Choose from: G-Technology 1TB G-DRIVE mobile USB-C Portable Hard Drive,G-Technology 2TB G-DRIVE mobile USB-C Portable Hard Drive.
Holding down the T key puts the Mini into 'firewire target disk mode':
http://support.apple.com/kb/HT1661
This mode has to be used carefully according to the protocol in the above link.
You'll need a fire wire 800-to-400 cable which will be 9-to-6, AFAIK -- assuming your Mini is an earlier-than-2009 model, and that the iMac has a firewire 800 port. If both computers are firewire 400, then just a plain 6-pin cable.
(Frankly, given the low cost of external drives, I can't see the value of using the Mini's (likely very small) hard drive for storage this way.)
http://support.apple.com/kb/HT1661
This mode has to be used carefully according to the protocol in the above link.
You'll need a fire wire 800-to-400 cable which will be 9-to-6, AFAIK -- assuming your Mini is an earlier-than-2009 model, and that the iMac has a firewire 800 port. If both computers are firewire 400, then just a plain 6-pin cable.
(Frankly, given the low cost of external drives, I can't see the value of using the Mini's (likely very small) hard drive for storage this way.)