Try the following and see if it helps. Click your way through the settings using this path: Settings > Transfers > Local Files > File Allocation (drop-down menu). In the drop-down select "Full Pre-Write". Doing this should force Tixati to fully allocate the disk space once a torrent download is initiated - in theory this should be contiguous disk space
i.e. it shouldn't be fragmented.
For more information on this see the webpage at the following link. (Please, note that the webpage isn't up to date with the latest version of Tixati, so the image looks different to what version of Tixati you might be using now, but you will, nonetheless, find information on this setting that will be of use to you):
http://support.tixati.com/settings%20-%20transfers%20-%20local%20files
It comes as something of a surprise to me that on that webpage Tixati recommends
not using this setting and regards it as "obsolete". Personally I find it a great setting - but then I download into and seed from a ramdisk and have no worries about how long it takes to fully allocate the file(s) - it happens pretty fast for me. I certainly hope that Tixtati do not remove this setting from future versions of Tixati.
In any case Lapetus I would recommend working your way through the "Help & Support" stuff that is available via the link at the top of the webpage you're reading now. It is actually helpful (though not necessarily fully up to date) and it isn't complete in the information that it provides but it is a good starting point for help.