A4000 hard disc

[Zac67]

You can hook up just about any SCSI HDD. There are wide SCSI (68 pin) and SCA ones (80 pin) but with an appropriate adapter 99% of them will work.

Drives / partitions larger than 4 GB will be a problem unless all your drivers and filesystems are made compatible:
http://thomas-rapp.homepage.t-online.de/4gb_faq.html
http://www.youngmonkey.ca/nose/articles/NewTekniques_9810/AmigaInMotion.html

[Zac67]

SCA drives (80-pin) do not offer any way of termination, nor do newer 68-pin ones (LVD/SE). So either get an adapter with terminator or - if you're planning to use several drives - a 68-pin cable complete with terminators (SE or LVD/SE) and an adapter for your 50-pin controller. Some 68-pin are not satisfied with 50-pin (narrow) termination and may not spin up.