Database Software

We are doing nightly hot backup to disk using RMAN. Here is our script:

configure controlfile autobackup format for device type disk to 'D:\backups\hotbackups\%F';
run
{
allocate channel dev1 device type disk format 'D:\backups\hotbackups\db-%U';
backup database;
backup archivelog all;
backup spfile;
release channel dev1;
}
crosscheck archivelog all;
delete expired archivelog all;
delete noprompt archivelog all completed before 'SYSDATE-2';

The script ran fine couple of nights ago, but the previous night we got the following error.

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on dev1 channel at 02/06/2008
21:38:01
ORA-19504: failed to create file "D:\BACKUPS\HOTBACKUPS\DB-4GJ82FHN_1_1"
ORA-27044: unable to write the header block of file
OSD-04008: WriteFile() failure, unable to write to file
O/S-Error: (OS 112) There is not enough space on the disk.
Recovery Manager complete.

I have checked that there is about 28GB free disk space, whereas the backup only requires about 14GB.
Any ideas/thoughts on what I should check for or what else could be wrong?

TIA
Pradeep.