DOS Batch file - Check for existence of a drive letter or directory

My backup solution runs a daily batch file to clear the previous weeks backup from a removeable drive that has a drive letter associated with it.

The problem is that if the drive is not inserted the batch file deletes itself.

How would I change the script to check if the drive letter is available and run, or exit if it is not available returning an error.

@ECHO OFF
REM Friday archive deletion using drive letter folder paths
i:
CD Backup_Vaults
REM Delete any subdirectories and included files
FOR /D %%i IN ("Servers_Backup_Vault\*.*") DO RD /S /Q "%%i"
CD Servers_Backup_Vault
REM Delete any other files in Servers_Backup_Vault
DEL *.* /S /Q CD..
CD Exchange_Backup_Vault
REM Delete any subdirectories and included files
FOR /D %%i IN ("Exchange_Backup_Vault\*.*") DO RD /S /Q "%%i"
REM Delete any other files in Exchange_Backup_Vault
DEL *.* /S /Q CD..
REM Delete any other files in Backup_Vaults
DEL *.* /S /Q CD..
EXIT