Turns out from an interesting thread on the SIMH mailing list, is that the Windows volume shadow copy service, and other ‘system’ protecting services may tag files like .dsk to be monitored, and to save a difference of every write to the file.

Yes that is right, on top of NTFS’s transaction log and rolls, you have the OS doing the same thing which in turn makes your disk access slower than hell.

The fix?

Quite simple, dont use TLA (three letter acronym) file extensions, or no extensions at all and suddenly you’ll find that your disk based emulation is significantly faster.