I thought this was Chris1331's issue ... Bob, did you wind up resolving matters for Chris?
As to your list Bob ... thanks for helping out ...
1. f/w upgrade ... done that (REV_1034) ... am now wondering if I can roll-back to REV_1028
2. chkdsk ... was able to do that up in a normal OS (Win XP Pro 32-bit SP3) with the WD as an externally-connected non-system drive from: Properties > Tools > Error-checking - Check Now (plus, if connected [externally] on a normal XP boot and errors are present, chkdsk will run on that drive anyways)
3. replacement ... not an option (due out-of-warranty, NLA & not really neccessary as the drive is only acting up in the BartPE environment with large file Writes)
4. reformatting ... not an option ... I have nowhere I can immediately move the existing 400~GB of data already on this single partition NTFS drive (my other 3 1TB WD's are all full of good stuff), plus, as for final comment in 3.
I'm focusing on the possibility that BartPE is loading an inappropriate driver for my eSATA controller - even though the appropriate driver (Si3112.sys - v18.104.22.168, 23-Aug-06) is configured properly in my BartPE build, is the latest from the controller vendor (Vantec UGT-ST350CB, seen by XP devmgmt.msc as 'Silicon Image SiI 3512 SATALink Controller') and is presumably matched to the BIOS of the controller.
The trick is to be able to bring an appropriate tool to bear within the BartPE environment to examine the actual driver being loaded.