Unless using a lot of tracks or higher samples rates, a 7200 HDD will do just fine.
You only need to have current projects on the ‘project’ drive, but you will want a rugged backup and archiving system (that is, process, software and hardware) to ensure you don’t lose completed ones in the cracks!
If using a large HDD for projects, use a sufficiently sized (that is, not too much larger than likely to be needed at most) partition as the first on the disk. This ‘short stroking’ will ensure that head travel is minimised.
Conversely, do NOT put ANYTHING in other partitions on the same drive that you will need during project use, otherwise you are FORCING excess head travel. Every ms of head travel means your drive cannot actually be transferring data.
You can use the other partitions on the same or other HDDs for backup copies of your current and old projects. The time taken for the extra head travel is not critical for these operations.
Format all media partitions with 64KB sectors. Even on HDDs, I found that copying several GBs of mixed sized files between drives with 64kB sectors to be up 30-50% faster than with the default 4kB sectors (for Windows). The downside with larger sectors is that if a file does not fill a sector, the remainder is wasted. Since multimedia files tend to be very much larger than 64kB, and only the last sector is unfilled, partitions/drives for those files only have 0.5% dead space, compared to 8% for an OS drive.