pe1900 perc 5i can't enable write back

I replaced a failed 5iR with a reconditioned perc 5i from ebay. The system is a pe1900 (I) with  a single 70Gb dell SAS.

On initial boot there were errors about the battery charge - I left the system to charge.

On booting again I did ctrl-R  and initialized the disk as RAID0 with default settings + "write back" (this was not shown as default) .

No errors or warnings this time.

I did (is this right ?) to get out of the virtual disk manager and to reboot

Then I installed Fedora 17 from a CD with out any obvious problems.

However on closer inspection

the system boot log concerning the drive says " Assuming drive cache: write through"

here's the rest :

 megasas: 00.00.06.14-rc1 Fri. Jan. 6 17:00:00 PDT 2012
 megasas: 0x1028:0x0015:0x1028:0x1f03: bus 2:slot 14:func 0
 megaraid_sas 0000:02:0e.0: PCI IRQ 142 -> rerouted to legacy IRQ 18
 megasas: FW now in Ready state
 megasas_init_mfi: fw_support_ieee=0
 megasas: INIT adapter done
 scsi2 : LSI SAS based MegaRAID driver
 megaraid_sas: fw state:c0000000
 megasas: fwstate:c0000000, dis_OCR=0
 scsi 2:0:4:0: Direct-Access     MAXTOR   ATLAS10K5_073SAS BP00 PQ: 0 ANSI: 5
 scsi 2:2:0:0: Direct-Access     DELL     PERC 5/i         1.00 PQ: 0 ANSI: 5
 142082048 512-byte logical blocks: (72.7 GB/67.7 GiB)
 Test WP failed, assume Write Enabled
 Asking for cache data failed
 Assuming drive cache: write through
 sd 2:2:0:0: Attached scsi generic sg1 type 0
 Test WP failed, assume Write Enabled
 Asking for cache data failed
 Assuming drive cache: write through
  sda: sda1 sda2
 Test WP failed, assume Write Enabled
 Asking for cache data failed
 Assuming drive cache: write through
 Attached SCSI disk

For want of a better idea, I repeated this process and got the same result.

Oh and hdparm gives the following:

/dev/sda:
SG_IO: bad/missing sense data, sb[]:  70 00 05 00 00 00 00 0b 00 00 00 00 20 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 multcount     =  0 (off)
 readonly      =  0 (off)
 readahead     = 256 (on)
 geometry      = 8844/255/63, sectors = 142082048, start = 0

I don't really know what this means and google gives nothing I can understand about the SG_IO line.

But I did leave the Read Policy at default "no read ahead" in the VDM.

I guess next I will try the "restore factory defaults" option in the VDM.

I am a software guy with a woeful hardware knowledge, can does any of this raise any flags for anyone ? Am I missing some step (quite possible) ? Are there any other diagnostics I can try ?

And just to reiterate : I did initially get an amber light and a battery not charged warning - but have not seen anything since.