Hi folks
If you are using the data on HDD for a server / streaming video / sending movies etc to things like remote TV's and firesticks then usually (assuming Network is fast enough) its better to use a 7200 RPM drive -- all other things being equal the build of the 7200 RPM ones are more suited to running 24/7 as per servers etc -- things like fans and bearings are usually much better engineered and have a considerably better M.T.B.F. (Mean time before failure for our "scientifically challenged colleagues here !!)

If you have any type of NAS running 24/7 or even for long periods before shutdowns go for the 7200 RPM drives especially if you "aggegate" drives or use any sort of RAID hardware / software.

The internal cache in an HDD is used to "pre-fetch" data by the OS when the OS is idle -- that way the I/O is overlapped with other operations -- and usually in Windows the "Pre fetch" algorithm is fairly good - especially if one does more or less the same tasks on a machine. So when the data is needed the computer can grab it straight from the HDD cache --usually pretty fast memory - especially compared with the speed of the HDD !!. Without caching and prefetching your machine would just wait each time I/O was needed -- you can get an idea of what it's like if you see an HDD light continuously on - especially say not enough RAM in system or trying to run too many concurrent tasks,

Cheers
jimbo