_FAT_cache_writeSectors decrease perfomance.

[z.g]

startting from revision 3543 and till current sequental writes of big amount of data (megabyte and bigger) take more time then before.
on ds in some case i have increase 1 mb write time from 2 seconds to 28 seconds.
one of speed decradation reasons as i can see is a reading from disk to cache before adding written data to cache.

[WinterMute]

This is one of the reasons why those changes have not yet been released as a stable binary and why we discourage using SVN builds. Having said that, do feel free to submit a patch