Incorrect unit allocation size. error comes when Tuning the Database Here I am Using the SQL IDERA tool for checking the Index and performance of the Database, then i found below error, can any one tell what is it talking about.


  Incorrect unit allocation size. Drive C: has a queue length average of
  7.26. This drive is also responsible for writing the transaction logs sequentially and performing the read/write of SQL Server pages which
  is typically random
  
  This results in latency currently measured at 0.0036ms as the head
  moves between sequential transactional log writing and the random
  Reading/Writing of SQL Server pages.  The unit allocation size has
  been detected to be 4KB.  SQL Server works far more efficiently with
  unit allocation sizes of 64KB and larger.
  
  Recommendation: It is recommended that for busy databases the
  Transaction logs reside on a different physical drive than the data
  files and that a unit allocation size of 64KB or larger be used.
For the first point:


  Incorrect unit allocation size. Drive C: has a queue length average of 7.26. This drive is also responsible for writing the transaction logs sequentially and performing the read/write of SQL Server pages which is typically random*



This indicates that some of the user databases are hosted on the C: Drive which as per the best practice should not be the case. It is preferred that mdf should be on a drive with its respective RAID level for higher reads medium writes, while LDF should be on a different drive apart from MDF. and per the error message it seems you're MDF and LDF reside on C drive: 


So check for those DB's whose files are on C: drive and move them to their respective data drives, if you have any, if not then get them attached. 


For allocation of unit size in SQL Server read  This article it will help:


And also as recommended and mentioned above in point a , you should move the heavy DB's files to their physical drives like D and L as per you're naming convention on the servers.