Thanks all for your messages. Here are, in a rather random order, some answers. I didn't want to give all the fine details because I feared it would have been to long a message to read...
- Sata Port/Cable : it's a portable PC, Dell E6400, so I don't think there is any choice here. Besides, the system works fine before the chkdsk (see below), I guess it's not H/W related.
- Unstable : yes sorry, it means here I get a wealth of various error messages, that I never get otherwise, plus Office 2007 won't launch (Word, Excel) any longer and Windows shuts itself down after a last "serious" error message. So totally unusable, and unstable...
- The chkdsk at boot *doesn't* display any error message. I might have more details in the system log (winlogon entry) but since Windows has gone unstable, I cannot access it... I think I tried also by booting from a WinPE CD, performed the chkdsk with /f /v options, there was a few minor errors, as usual, except one is serious.
The strange thing is that before the chkdsk in invoked, my system is rock solid, never a BSOD or any crash, hang, freeze.
I will try next week a clone in file mode with Casper, my last option today.