DB Engine :: BACKUP LOG Command Failed - Device Is Not Ready

When I execute the following transaction log backup TSQL in SSMS

BACKUP LOG ProductDB 
TO DISK = 'D:Database_BackupTransProductDB.trn';

I get this error:

Msg 3201, Level 16, State 1, Line 2
Cannot open backup device 'D:Database_BackupTransProductDB.trn'. Operating system error 21(The device is not ready.).
Msg 3013, Level 16, State 1, Line 2
BACKUP LOG is terminating abnormally.

I also have a daily database backup job that puts the backup file under this directory.

D:Database_BackupData

So I changed the path to this and it still fails.

BACKUP LOG ProductDB 
TO DISK = 'D:Database_BackupDataProductDB.trn';

However if I change it to this path, it works:

BACKUP LOG ProductDB 
TO DISK = 'MyServerDataProductDB.trn';

This turns out to be the path I used a few weeks back when I backed up the log for the very first time.  But I now wanted to store my transaction log backups in D:Database_BackupTrans.