Too many programs make a bad assumption about how long a disk operation will take. That assumption is

The amount of time left in an operation is equal to the total remaining bytes of operation left divided by the dividend of the total bytes completed and the amount of time spent on completing it.
The assumption is, in many cases, plain wrong.

Generally, copying n one byte files takes far longer than copying one n-byte file. Disk seeks are costly. Often file creation and finding is slow as well. For example, finding and creating files over WebDAV is much worse than read or write streaming.

To accurately measure the time to complete an operation, you must first figure how much overhead there is per file. Then add in the overhead in the calculation. This still isn't perfect, but it's much closer than most programs use.

Many disks don't run at the same speed at the inner and outer track. Disks have multiple platters and heads, and can sometimes use many at a time. Seek times vary based on current head position. The only thing about all predictions of the future is that they are all wrong, etc.

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