cannot mount cdrom - not a newbie problem

[Timothy Smith]

i have a genuine problem here.
i noticed my backup cdrw's had stopped working at some point. upon 
furthur investigation i find i cannot mount cd's full stop.

titan# mount -t cd9660 /dev/acd0c /mount
cd9660: /dev/acd0c: Invalid argument

the above command makes the drive light flicker for 3 seconds then the 
error.

this is NOT a hardware problem. i get the exact same issue with my brand 
new dvd drive.

here is what dmesg gas to say:
acd0: DVD-R <HL-DT-ST DVDRAM GSA-4160B> at ata1-slave WDMA2
da0 at hpt3740 bus 0 target 0 lun 0
da0: <HPT3xx RAID 1 Array 3.00> Fixed Direct Access SCSI-0 device
da0: 190782MB (390721957 512 byte sectors: 255H 63S/T 24321C)
Mounting root from ufs:/dev/da0s1a
cd0 at ata1 bus 0 target 1 lun 0
cd0: <HL-DT-ST DVDRAM GSA-4160B A301> Removable CD-ROM SCSI-0 device
cd0: 16.000MB/s transfers
cd0: cd present [1 x 2048 byte records]

note this was working perfectly.
i suspect that some how my ata device has gone "bad" some how.

let me get the obvious replys out of the way:
yes i have googled nothing i can find has even touched on a fix.
yes there is a disc in the drive
no it is not an audio disk.
there is nothing else connected to the ide ports.