hmmm ... I believe that it it still a best practice to separate out the following to separate disk drive letters:

- database data

- database transaction log

- tempdb

- system databases

- database full backups

- database differential/t-log backups

Note: you should also have separate internal drive for the OS, the system pagefile and application installation software.

Your SAN administrators may think it is OK, but really it is not.

More drive letters mean more I/O paths from Windows to the storage - more I/O paths implies and should create more throughput. This is provided the HBAs, fabric and storage frame can handle it.

The only way that the 'one drive' scenario begins to make sense is if you are using 'mount points' for your SAN storage within Windows.

There can be a rather indepth discussion when it comes to RAID levels. Once you decide upon a drive strategy then it would be time for the RAID level discussion.