Just sharing a Scenario where we had to reset the index for FAST search & we faced some issues.
How to Reset Index for FAST Search for SharePoint 2010 ?
Log on to SharePoint Central Administration --> Fast Search Content service application
--> Index Reset
Wait for the confirmation.
Don't start any crawls yet.
Now go to Fast Search server & enter the following command:
Clear-FASTSearchContentCollection -Name sp (sp is the fast collection in our case)
wait for the command to finish as it can take some time.
In our case this has failed so we have to use the below procedure but if above is success in your case then stop as index has been reset & you can continue crawling again.
Stop the Fast search services on all fast search servers:
Rename or delete data_fixml & data_index directories (we renamed it first & than deleted afterwards)
Start the services again, these folders will get recreated.
Now you can start the crawling again as index has been reset.
You can also check if index has been reset or not by following command
Get-FASTSearchContentCollection -Name sp
In our case as we had only sp collection so number came out to be zero.