Hello Luzie!
So this feature only for loading iSDOS/TASiS operation system (it not compatible with FAT and have itself FAT format) from HDD.
You can read about iSDOS\TASiS hier:
http://atmturbo.nedopc.com/projects/os_prj.htm (use google translate to translating from russian).
Maksagor is chief of TASiS project, you can ask him direct.
Easy way to using HDD:
1) Format it to FAT32 (on PC)
2) Copy some *.trd, *.scl diskette-images files (for example from vtrdos.ru)
3) Start it from "File browser" on ZXEVO
But I think - using SDcards easest way
LVD: дай челу торрент-ссылку на vtrdos.ru контент.