Q: How can I make this hard disk usable?
After you have installed you hard drive, you need to partition and format it. The best utility available is WUTIL. The latest version is 3.2. This freely available program allows for initializing and low-level formatting of any type of MFM hard disk (provided that you know its characteristics) and the subsequent partitioning. WUTIL can create bootable partitions for CP/M-86/80, MS-DOS, Concurrent CP/M-86, and VENIX/Rainbow. It cannot create a p-System partition. WUTIL is available in the archives on this site.
Remember, however, that if you have a Rainbow 100A, you will not be able to boot from the hard disk.
If WUTIL returns errors while initializing your hard disk, do not panic. If your hard disk is over 20 megabytes and you are using DEC's HD cable, you may be experiencing a known issue.
Digital also had it's own initializing program, called the Winchester Utility. You should probably forget about Digital's own and use WUTIL, which is much more versatile.